What does Matthew 11:18 mean?

Demanding, immature children playing in the marketplace are the image Jesus has just applied to the Israelites of His generation. Children complain because the other children won't "play the game" they want to play, or when the game is not being played exactly as they prefer. Here, Christ begins to show how the people of Israel, in this moment, are like those children. They have expectations for how a prophet should act. No matter what they see or hear, they'll demand something different.

For example, instead of hearing John the Baptist's words and truly taking them to heart, they accuse him of being oppressed by demons. Some demon-possessed people would isolate themselves from the community, dress strangely—if at all (Mark 5:14–15)—and even hurt themselves. John lived in the wilderness, dressed in uncomfortable camel hair clothes, and ate only locusts and wild honey, refusing to touch alcohol (Luke 1:15). By society's standards, he was a bizarre character.

John the Baptist was not actually demon-possessed, of course. The problem was that he didn't meet the expectations of many Israelites for the one preparing the way for the Lord. They used this as an excuse to ignore John's harsh warnings about God's coming judgment. Instead of repenting, they accused him of being possessed by evil spirits.

Jesus will point to a different accusation in the next verse, one exactly opposite this criticism of John the Baptist (Matthew 11:19). Matthew will report in the following chapter that Jesus was also accused of being associated with Satan.