See Matthew 17:1-9. This experience is sometimes called “The Transfiguration.” Also see the first chapter of 2 Peter in which the apostle Peter, just before his death, writes of the Transfiguration in the context of writing about the meaning of Jesus’ life for all of us.
Moses and Elijah represented the previous height of Jewish achievement with God. Jesus, being God in the flesh, is thus depicted by this event as an even greater achievement. Thus the command to “Listen to Him!”