They were speaking
Of his departure
Jesus was to accomplish this
οἳ ὀφθέντες ἐν δόξῃ ἔλεγον τὴν ἔξοδον αὐτοῦ, ἣν ἤμελλεν πληροῦν ἐν Ἱερουσαλήμ
Luke said that the 3 men, Jesus, Moses, and Elijah, all appeared in their glory (οἳ ὀφθέντες ἐν δόξῃ). They were speaking about the departure of Jesus (ἔλεγον τὴν ἔξοδον αὐτοῦ) and how it was about to be accomplished (ἣν ἤμελλεν πληροῦν) in Jerusalem (ἐν Ἱερουσαλήμ). This is a unique statement by Luke, since the other synoptics did not mention this. Luke said that these 3 transfigured men, Moses, Elijah, and Jesus, talked about the future departure of Jesus from Jerusalem, in other words, his upcoming death. Have you ever discussed your future death with anyone?