What does Matthew 16:6 mean?

This comment comes during a trip across the sea of Galilee (Matthew 16:5), following another a confrontation between Jesus and His critics. Earlier, a group of Pharisees and Sadducees approached Him to demand a miracle. He responded that He'd give them no more signs, other than the sign of Jonah, referring to His own death and resurrection (Matthew 16:1–4). The disciples, meanwhile, realize that they have failed to pack bread for this trip. Whether they say so to Him, or not, the men assume that Jesus' reference to leaven is in response to their concern over bread.

This, however, is not what Jesus means. Yeast was often used as a metaphor for evil in Jewish teaching. Not because it was unclean, or inedible, but because yeast begins as a very small thing, growing larger and larger within the bread. A tiny amount of yeast, relatively speaking, can affect a large quantity of bread. This is why the Old Testament law sometimes required Israelites to eat unleavened bread: bread with no yeast.

Christ is giving a warning about just how dangerous the errors of the Pharisees and Sadducees really are. Following those ideas could lead to enormously wrong beliefs for the people. Given that they're thinking about edible bread, this is not the interpretation the disciples first assume.