Mark 9:1 And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power.
The meaning of this is that some (which, in context, we find to be Peter James and John) would not die until they had seen the kingdom of God.
The rest of the narrative explains how this came about.
Mark 9:2 And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them. 3 And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them. 4 And there appeared unto them Elias with Moses: and they were talking with Jesus.
Luke tells that the appearance of Jesus changed, and Moses and Elijah appeared in glory.
Luke 9:28 And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray. 29 And as he prayed, the fashion of his countenance was altered, and his raiment was white and glistering. 30 And, behold, there talked with him two men, which were Moses and Elias: 31 Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem.
Jesus explains that it was a vision.
Matthew 17:9 And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
Putting these facts together we see that Peter James and John were given a vision of the future kingdom of God.
The kingdom of God will be established when Christ returns to the earth. At that time Jesus will appear in his glory and Moses and Elijah will be glorified along with the saints of all ages.
The significance is that for Moses and Elijah to appear in glory it meant that Jesus would accomplish his sacrifice (otherwise they could not be glorified). We might reasonably suppose that they spoke of Jesus’ decease (literally ‘exodus’) as a thing already accomplished; for he also was glorified.
Peter speaks of the experience.
2 Peter 1:16 For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty. 17 For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. 18 And this voice which came from heaven we heard, when we were with him in the holy mount.
In conclusion: Peter, James and John saw the kingdom of God come with power in a vision on the mountain.
Peter, James and John are, of course, now dead awaiting resurrection at the return of Christ.
I hope you find this helpful.