Peter, James, and John were helped in their understanding of who Jesus was by the events on the mountaintop. They witnessed a change in Jesus' body. They saw him clothed in white as was the Divine One described by Daniel (7:9). And they saw Moses and Elijah, great figures in the history of their people, both who had spoken the word of the Lord.
If you keep reading Mark 9:
Then a cloud overshadowed them. And from the cloud came a voice.
What Jesus has known (1:11) is now told to these disciples: The voice tells them, "This is my Son, the Beloved; listen to him!"
And what he first tells them is not to tell anybody else until after the Son of Man had risen from the dead. Fred Craddock in Preaching through the Christian Year B, explains this command: They may have heard that Jesus is the Son, the Beloved, but they haven't yet heard everything they need to know to be effective disciples. They are not yet ready to be witnesses nor are their audiences yet read to hear it.
Off on a tangent: Look back at Exodus 19:16-20 that tells of the appearance of a thick cloud on a mountain and the voice of the Lord; also Exodus 24:12-18, Moses and Aaron go up the mountain, the glory of the Lord appears in the cloud, then Moses enters the cloud. Other references to the cloud as a symbol of the divine presence include Numbers 14:10; Ezekiel 1:4; Daniel 7:13-14; Mark 13:26; 14:2. (Thank you once more, Allen & Williamson, for your Preaching the Gospels.)
All the elements of Mark's account are there: the six days of waiting, the cloud, the glory, the voice, the descent from the mountain. Moses' face shone due to his experience in the presence of God. Exodus also describes the making of the tent of meeting.