This is an account of the transfiguration. Matthew, Mark and Luke have an account of this same event. St Peter in his second letter have an account of this same event. We don't really know what happened. We have the details in today's gospel. But they do not tell us much. They are forceful in a sense because they tell us something profound. Something wonderful. Something beautiful. Something filled with peace occurred. This pressed itself deeply on their memories.
The Transfiguration is a pivotal moment in the synoptic gospels. The disciples raise their eyes. The Greek word is 'monos'. Jesus is there alone. I would suggest that the word 'Monos' has a sense of foreboding about it. It is ominous. Jesus must enter into his aloneness. His loneliness. He must make the journey to Jerusalem. No one else can do this for him.