After predicting his death in Jerusalem (16:21-23) and a conversation about "taking up your cross," Jesus takes Peter, James and John up on a high mountain. Peter, James, and John are the inner circle of the Apostles. He frequently takes them aside and does extra work with them that is not afforded to the other 9 Apostles. There are conflicting theories as to where this mountain is. Some place it at Mt. Tabor above the Sea of Galilee, others place it at Mount Hermon in the north -- not too far from Caesarea Phillipi (16:13). The location is less important than the experience. On the mountain, with the inner leadership circle (Peter, James and John) watching Jesus true spiritual nature is revealed. The outside form assumes the appearance of the inside reality. Moses and Elijah represent the sum of the Old Testament -- Moses represents the Law and Elijah, considered to be the greatest of the Old Testament prophets, represents the prophets. The Hebrew bible was often referred to as "The Law and the Prophets" when referred to in its totality.
Moses and Elijah are talking with him -- the details are not given but one of the other gospels notes that they were talking with Jesus about his impending "departure" at Jerusalem -- that his Jesus impending death. Peter's response is typically human. Not knowing what he is say, he suggests that they build a somewhat permanent shrine on the spot. Peter wants to build tabernacles, tents, for the three luminous figures. Then the cloud shows up. In the Bible whenever a cloud shows up it is God and sure enough the voice of God speaks to those on the mountain. I have often thought that the voice from the cloud sounds a bit impatient, especially when it says "listen to him!" It feels like one of my school teachers looking at me and saying "Pay attention!" (because, of course, I wasn't). Peter, James and John are reminded that Jesus is who he says he is and that they should stop worrying about building booths and other temporary nonsense and listen.
Much is made of the story of Jesus curing the boy with the Demon (14-21) but it seems simple enough to me. The Christian life is a spiritual journey. We are called to go on to maturity (to grow up spiritually). It stands to reason that the spiritually immature disciples are encountering things they cannot handle. When Jesus tells them to have "faith like a grain of mustard seed" all he is saying is they should mature to a point of having a faith (however small it may seem) that is unshakable and immovable and will continue to grow.