Read Mark 9:2-9
2 Six days later Jesus took with him Peter, James, and John, and led them up a high mountain, where they were alone. As they looked on, a change came over Jesus, 3and his clothes became shining white—whiter than anyone in the world could wash them. 4Then the three disciples saw Elijah and Moses talking with Jesus. 5Peter spoke up and said to Jesus, “Teacher, how good it is that we are here! We will make three tents, one for you, one for Moses, and one for Elijah.” 6He and the others were so frightened that he did not know what to say.
7 Then a cloud appeared and covered them with its shadow, and a voice came from the cloud, “This is my own dear Son—listen to him!” 8They took a quick look round but did not see anyone else; only Jesus was with them.
9 As they came down the mountain, Jesus ordered them, “Don’t tell anyone what you have seen, until the Son of Man has risen from death.”
Moses and Elijah are talking with Jesus. Seeing this sight confirms that Jesus is who he says he is. Moses is the first prophet who points Israel so clearly to God; Elijah calls them to see God clearly. In this conversation it seems that the two approve Jesus.
Finally - the voice from heaven confirms:
"This is my Son."
When I understand that you are God, and that you love me, I have the power to live out my calling in the world.