What does Matthew 11:15 mean?

This statement sounds like a platitude or a figure of speech that would be tacked on to a wise saying: "He who has ears to hear, let him hear." When Jesus says it, though, it is a clue to go back and look at what He has just said. Those with ears should review Jesus' words, because He has just given them a clue that His prior statements are important.

Those listening only on a surface level may have only heard Jesus praise John the Baptist and hold him up as the fulfillment of the prophecy that Elijah would return (Malachi 4:5). Yet, in saying this, Jesus has really claimed to be the Messiah Himself. He's saying that John the Baptist prepared the way for Him. After all, Malachi's prophecy was God saying that "I will send you Elijah the prophet before the great and awesome day of the LORD comes." That means that Jesus is claiming the title of Lord and announcing that He has arrived to bring about the day of the Lord.