Matthew 16:28 "Assuredly, I say to you, there are some standing here who shall not taste death till they see the Son of Man coming in His kingdom."
This is followed in chapter 17 by Jesus’ transfiguration.
Matthew 17:1 Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves; 2 and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. 3 And behold, Moses and Elijah appeared to them, talking with Him.
In Mark’s gospel the two events are kept together.
Mark 9:1 And He said to them, "Assuredly, I say to you that there are some standing here who will not taste death till they see the kingdom of God present with power." 2 Now after six days Jesus took Peter, James, and John, and led them up on a high mountain apart by themselves; and He was transfigured before them. 3 His clothes became shining, exceedingly white, like snow, such as no launderer on earth can whiten them. 4 And Elijah appeared to them with Moses, and they were talking with Jesus.
Similarly, in Luke’s gospel:
Luke 9:27 "But I tell you truly, there are some standing here who shall not taste death till they see the kingdom of God." 28 Now it came to pass, about eight days after these sayings, that He took Peter, John, and James and went up on the mountain to pray. 29 As He prayed, the appearance of His face was altered, and His robe became white and glistening. 30 And behold, two men talked with Him, who were Moses and Elijah, 31 who appeared in glory and spoke of His decease which He was about to accomplish at Jerusalem.
The way that I understand it is that in the transfiguration the disciples see a vision of the future kingdom where Jesus glorified speaks with Moses and Elijah. They discuss Jesus’ decease (exodus) an event still future at that time. Moses, we know is dead and buried and will not live again until he is raised from the dead in the kingdom of God. For Moses to be seen alive is proof positive that the sacrifice Jesus was due to make had been successfully accomplished in the time represented in the vision. I must therefore conclude that the vision is the fulfilment of Jesus’ promise that some of his disciples (namely Peter, John, and James) would live to see him in his future glory in the kingdom of God.
I hope you find this helpful.