Our Lectionary tells us that today's Gospel is from Mark, Chapter nine, verses two to ten. However, our Lectionary may be at fault in leaving out the first part of verse two. In telling us that Jesus took Peter James and John with him, it leaves off Mark having written that this happened "after six days."
What had happened six days before was that for the first time, Jesus foretold that he was going up to Jerusalem where the leader would bring about his disgraceful death.
You will remember that when Peter said he would not let that happen, Jesus said, "Get behind me, Satan."
Anyway, Mark wrote this Gospel story to show us that Our Lord's imminent death made him so sad that he needed to be alone with the Father on the mountain.
In the imagination of Peter, James, and John there was a wide sheet separating heaven from earth, and they saw that sheet stretched down so that the fame of Jesus was engulfed in heavenly glory.
Jewish tradition held that Elijah and Moses were the only two mortals already in heaven, so Peter, James and John imagined they were seeing those two wander over to chat with Jesus.
This transfiguration of Jesus into heavenly glory was also meant to tell Peter, Jame and John that heaven was waiting for them. It was meant to tell us the same thing.