‘They were speaking of his departure, which he was about to accomplish in Jerusalem.’
And for the disciples, sleepy and confused, it was the beginning of a departure for them, too, the beginning of an exodus - the word Luke actually uses in this story. They had seen something that drew them away from what they were used to; they had seen into a depth and mystery in the life of Jesus that would take them to a very distant place. After he resurrection, Jesus says to Peter that one day his discipleship will mean that he is led where he doesn’t want to go. He too must complete his exodus, his journey to the cross and beyond.