Thanks for your question.
Here is an explanation for you taken from the following online Christadelphian commentary:
Here is the perfect picture of unity -- a DIVINE UNITY: the sharing of the blessings of God, poured out in love upon us all. How foolish to suppose that Biblical fellowship can be enjoyed by those who do not share the blessing of a common hope! But how foolish also to suppose that God views favorably any division among those who, despite minor differences, do share a common hope!
This psalm might well bear the title: "The Descent of Divine Blessings". The repetition of this word "descend" is obscured in the AV because, oddly enough, its three occurrences are translated three different ways: "ran down" and "went down" as well as "descended". But mark its uses here, and then we may follow the lovely refrain:
"Unity is like the precious ointment that DESCENDED upon Aaron's beard;
Yea, that DESCENDED even to his skirts;
Even like the dew of mount Hermon that DESCENDED upon Zion."
Through this refrain, as with the gently descending rains of summer, our Father seeks to cleanse our hearts of all pride and boasting. He seeks to refresh in our minds the principle that our unity is derived from above (descending from God), and is not the product of our own labors. Surely James had this in mind when he wrote of "the wisdom that is from above... pure, peaceable, gentle, easy to be entreated, full of mercy... and the fruit of righteousness is sown in peace of them that make peace" (Jam 3:17,18).
"Can it be a question whether brethren should from conflict cease?" Truly unity -- with God at the center -- leads inevitably to righteousness, peace of mind, and the solution of those perplexing tangles in which Christ's brethren, as they execute their duties, so often find themselves.
"Behold how good and how pleasant..." It is not simply good (right and proper) that we dwell together in unity. It is also pleasant (exceedingly delightful) that we do so. The appeal of Abraham to his kinsman Lot might as well be an exhortation to us: "Let there be no strife, I pray thee, between me and thee... for we be brethren" (Gen 13:8). It should be enough to encourage our endeavors in the direction of unity, if only it were right in God's sight. But the "icing on the cake" is that it is superlatively delightful as well. There is no state on earth to match this dwelling together in "the unity of the Spirit" (Eph 4:3) -- with shared feelings and purposes -- whether it be as husband and wife, or as an entire family, or in the larger "family" of the ecclesia.
The brethren of Christ "dwell together". This does not mean that they merely meet and socialize a little before and after ecclesial functions. This means instead a continuing together, in the closest communion. As David exhorted the men of Judah, "Ye are my brethren, ye are my bones and my flesh" (2Sa 19:12). And if other duties or infirmities or distances make this personal "dwelling together" impractical, then we still have recourse to thought and prayer for one another as a means of achieving this union.
The goal, the focal point of our unity, is mount Zion and her king. We could have no unity but for Christ and the promises. This Psa 133 is one of the fifteen "Songs of Degrees". These glorious songs are centered in the worship of the Lord in His temple, and are in part prophetic of the kingdom age. They are also called "Pilgrim Songs" and "Songs of Ascents" -- because they appear to have been composed with those in mind who journey upward to worship the Lord of Hosts in the elevated mount Zion.
And so we find the expression of our unity in the ascending of our common petitions and the descending of our common blessings. True brethren of Christ are united in one hope, one need, and one experience. Unity with Christ is intimately bound up with unity with our brethren. We cannot have one without the other.
"Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me" (Mat 25:40).
But when we exalt ourselves above our brethren then we do what our Lord and Saviour would not do; we displease him; and we endanger our own oneness in the "Body of Christ".
We must now examine the beautiful comparisons of this Divine unity: first, the holy anointing oil of Aaron (Psa 133:2), with which the tabernacle and its furniture were also anointed (Exo 30:23-33).
This anointing served as the consecration of God's priests, empowering them to fulfill the duties of their office. All priests were anointed, but the high priest received a great abundance, an overflowing "to the skirts of his garments". The anointing oil was an expression of God's love in His ordinances: its "pouring out" prompts the "virgins" to love Him (Song 1:3). The anointing of Aaron pointed forward to that of Christ -- "with the oil of gladness above his fellows" (Psa 45:7). Christ has received a greater glory and a greater strength, so that he might be a perfect High Priest to us his brethren.
The "oil" descended first and in greater measure upon the head, Christ, and then descended to the skirts of his garments, his "Body" as well! God's special provision of a Son in whom we might be reconciled to Him (2Co 5:21) calls forth this picture of oil covering, not just the head, but the whole body! None of us is the head, only Christ. A realization of this simple fact would curtail most ecclesial wrangling and self-seeking. Our unity is that each of us is an integral part of the one, undivided "body of Christ" (1Co 12). All of the body shares the blessings that came through the special anointing of the head. It is not the believers who make this unity possible, for Christ has already done so. Neither do we earn our place in "fellowship" around the table of the Lord. We merely accept it, and rejoice in the blessings it affords. Our righteousness is not our own, but his that called us. Our unity is based upon our abject humility and our sincere acceptance of our subordinate and dependent position.
Finally, in v 3, we are called upon to consider the descending dew of Hermon as an allegory of our unity. The anointing oil has brought to our minds the death of Christ ("for in that she hath poured this ointment on my body, she did it for my burial" -- Mat 26:12). Here, the dew stands foremost as a symbol of resurrection (his, and ours in prospect) -- completing the cycle.
Hermon is a range of three peaks which dominates the northern parts of Israel, and which is visible over the entire Land. Because of its great height, Hermon is covered with snow most of the year. In late summer this snow begins to melt, the runoff feeding reservoirs and springs to supply water to the thirsty lands of the south.
The obvious and intended symbolism of Hermon is that of the Divine Blessing, stored up in the providence of God until the proper time, when it is most needed. We can read v 3 by omitting the phrase beginning with "and" which is in italics. Thus: "As the dew of Hermon that descended upon the mountains of Zion..." It is the same dew although the two areas are over 100 miles apart: Observers say that the breezes that blow from the north across the snow-covered face of Hermon bring cooling moisture as far south as Jerusalem. (Perhaps it is this very phenomenon which called forth the description of a faithful messenger -- "as the cold of snow in the time of harvest": Pro 25:13.)
The dew comes as God's blessing during the hottest part of the year. We are told the king's favor is as dew upon the ground (Pro 19:12), and especially upon the hallowed ground of Zion (Bible symbol of the unified body of saints: Heb 12:22; Rev 14:1). The "manna" of God -- the bread of His blessing that descended each day -- was said to come as the dew falling upon the ground (Num. 11:9), emblem of the unfailing compassions of God for His people, renewed each morning (Lam 3:22,23).
"For there [upon the mountains of Zion] the Lord commanded the blessing, even life for evermore." Finally, and most important, the dew portrays the saints on the glorious resurrection morn: "O dwellers in the dust, awake and sing for joy! For thy dew is a dew of light" (Isa 26:19, RSV)... the first rays of the "Sun" revealing a glorious spectacle of reflected light, dancing upon the grass: "From the womb of the morning like dew your youth will come to you" (Psa 110:3, RSV).
In that greatest of all days the Lord will achieve that perfect unity of which our assemblies are only a shadow. A great multitude, which no man can number, born in a day, silently, irresistibly, as the dew of heaven. Such is the picture of the development of the body of Christ, a multiplicity and yet a unity, appearing all together for the first time.
In the memorials which we receive each Sunday we have the reason for our unity. We have the "bread which strengtheneth man's heart" (Psa 104:15) and the "wine that maketh glad the heart of man". The strength and joy of a renewed life through Christ come from above. They descend upon us as the precious anointing oil, as the dew upon the earth beneath; and all we need do is stretch forth our hands and hearts to receive the eternal bounty of God's inexhaustible store.
Let us draw together in this, the true unity of shared blessings. Let us look upon each of our brothers and sisters with deeper understanding. We are all of us, collectively and individually, those "for whom Christ died". Let us be silent and reverent as we stand with our brethren and mingle our prayers like incense on the altar before the throne of heaven. We have come into the miraculous presence of God's grace. It is His mercy alone that has brought each of us to share the oil of healing and the dew of refreshing, the bread of strengthening and the wine of joy. In the loving provision of His Son, the all-powerful, self-existent Lord of all being has commanded the blessing of life for evermore; and none can turn aside His decree.
May we grasp and cherish that bright hope, and lead lives worthy of the Saviour who loved us all. May the contemplation of that awesome work, the reconciliation of sinners to God, transform our minds in the understanding and practice of true Biblical fellowship. And may we now -- and forever -- "dwell together in unity" under the shadow of His love. -- George Booker
I hope you have found this helpful.