It's easy to overlook one important aspect of the biblical narrative of the Transfiguration. Jesus was transfigured while he was praying.
Pope Benedict XVI pointed out this reality in an Angelus message in 2013.
[W]e have a particularly beautiful Gospel, that of the Transfiguration of the Lord. Luke the Evangelist highlights in particular the fact that Jesus was transfigured while he was praying.
He goes on to explain that this is an important lesson for all of us to meditate on.
In meditating on this passage of the Gospel, we can learn a very important lesson from it: first of all, the primacy of prayer, without which the entire commitment to the apostolate and to charity is reduced to activism. In Lent we learn to give the right time to prayer, both personal and of the community, which gives rest to our spiritual life. Moreover, prayer does not mean isolating oneself from the world and from its contradictions, as Peter wanted to do on Mount Tabor; rather, prayer leads back to the journey and to action.
It was after the Transfiguration that Jesus was led to offer himself as a sacrifice on the cross.
Prayer should always have a primacy in our lives, and the Transfiguration reminds us of that central truth.
