Matthew 16:28 Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
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.
Luke 9:27 But I tell you of a truth, there be some standing here, which shall not taste of death, till they see the kingdom of God.
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.
Matthew 17 explains how this came about.
Matthew 17:1 And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart, 2 And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light. 3 And, behold, there appeared unto them Moses and Elias talking with him. 4 Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias. 5 While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him. 6 And when the disciples heard it, they fell on their face, and were sore afraid. 7 And Jesus came and touched them, and said, Arise, and be not afraid. 8 And when they had lifted up their eyes, they saw no man, save Jesus only. 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.
Jesus tells them it is a vision.
In the vision they see Jesus glorified Moses and Elijah, also glorified, conversing with him.
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.
Their conversation was concerning Jesus’ death at Jerusalem
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 (Greek – exodus) which he should accomplish at Jerusalem.
From the account. we understand 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 of the vision is that for Moses and Elijah to appear in glory it meant that Jesus’ sacrifice would be successfully accomplished (otherwise they could not be glorified).
We might reasonably suppose that they spoke of Jesus’ decease as a thing already accomplished.
Peter later writes 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, in a vision on the mountain, ‘the Son of man coming in his kingdom’.
This coming is, of course, still in the future.
Peter, James and John are now dead awaiting resurrection at the return of Christ.
I hope you find this helpful.