What does Matthew 17:11 mean?

The disciples are trying to figure out how the prophecy about the return of Elijah the prophet before the coming of the Messiah (Matthew 17:9–10) fits with what they have just seen on the mountain (Matthew 17:1–3) and with Jesus' mention that He will be raised from the dead. Jesus has confirmed that the teaching about the prophecy is true. It is found in Malachi 4:5–6:

"Behold, I will send you Elijah the prophet before the great and awesome day of the LORD comes. And he will turn the hearts of fathers to their children and the hearts of children to their fathers, lest I come and strike the land with a decree of utter destruction."

Now Jesus agrees that Elijah will come and that he will restore all things. Jesus also says in the following verse that Elijah has come, meaning that John the Baptist was the fulfilment of the prophecy about the return of Elijah. While John was not literally a reincarnation of Elijah (John 1:20–21), and many people rejected John's message (Matthew 11:14), he was successful in leading many in Israel to repentance (Luke 1:17). Ultimately, however, his work ended with rejection by Israel's religious leaders and execution by Israel's political leadership (Matthew 14:1–12).