The transfiguration of Christ is one of the central events recorded in the gospels. Immediately after the Lord was recognized by his apostles as "the Christ [Messiah], the Son of the Living God," he told them that "he must go up to Jerusalem and suffer many things ... and be killed and on the third day be raised" (Matthew 16).
The announcement of Christ's approaching passion and death was met with indignation by the disciples. And then, after rebuking them, the Lord took Peter, James, and John "up to a high mountain"—by tradition Mount Tabor—and was "transfigured before them."
... and his face shone like the sun, and his garments became white as snow and behold, there appeared to them Moses and Elijah, talking with him. And Peter said to Jesus, "Lord, it is well that we are here; if you wish I will make three booths here, one for you and one for Moses and one for Elijah."
He was still speaking when lo, a bright cloud overshadowed them, and a voice from the cloud said, "This is my Beloved Son, with whom I am well pleased; listen to him." When the disciples heard this, they fell on their faces with awe. But Jesus came and touched them, saying, "Rise, and have no fear."
And when they lifted up their eyes, they saw no one but Jesus only. And as they were coming down the mountain, Jesus commanded them, "Tell no one the vision, until the Son of Man is raised from the dead" (Matthew 17).
In the Transfiguration, the apostles see the glory of the Kingdom of God present in majesty in the person of Christ. They see that in him, indeed, all the fullness of God was pleased to dwell, that "in him the whole fulness of deity dwells bodily" (Colossians 1:19, 2:9). They see this before the crucifixion so that in the resurrection they might know who it is who has suffered for them, and what it is that this one, who is God, has prepared for those who love him. This is what the Church celebrates in the feast of the Transfiguration.
— Fr. Thomas Hopko, The Orthodox Faith (read more)