The three disciples have accompanied Jesus to the mountaintop. There, they witnessed something that terrified them.
Then a cloud overshadowed them. And from the cloud came a voice.
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.)
Another tangent: What Jesus has known is now told to these disciples: The voice tells them, "This is my Son, the Beloved; listen to him!" Compare this with what Mark tells us that the voice said at Jesus' baptism, "You are my Son, the Beloved; with you I am well pleased " (1:11).
Sing (or, at least read) these two hymns about the Transfiguration: 258, "O Wondrous Sight! O Vision Fair" and 260, "Christ, upon the Mountain Peak," The United Methodist Hymnal.