What did the disciples see and hear that day? There is a lot of theological and mystical speculation provided over the millennia, especially by the Fathers but let us put all that aside and wonder simply about what it must have been like to be there.
It is a sensual experience; though mystical and miraculous, it is first and foremost sensual - we see the brightness of Jesus' clothes; we see Moses and Elijah beside him; we hear the voice of God; we feel the overwhelming emotions.
Peter's take is pretty straight forward here and leaves little to the imagination. Jesus is the Christ and the Father has told us so. He tells us this story and the evangelists recall it not to deceive us but to illuminate.
Bathe for a few moments in that Taboric Light.
We did not follow cleverly devised myths when we made known to you the power and coming of our Lord Jesus Christ, but we had been eyewitnesses of his majesty.
For he received honor and glory from God the Father when that unique declaration came to him from the majestic glory,
"This is my Son, my beloved, with whom I am well pleased."
We ourselves heard this voice come from heaven while we were with him on the holy mountain. Moreover, we possess the prophetic message that is altogether reliable.
You will do well to be attentive to it, as to a lamp shining in a dark place, until day dawns and the morning star rises in your hearts.
-- 2 Peter 1:16-19