What does Matthew 15:12 mean?

Jesus has just said to the people gathered around Him that it is not what goes into the mouth that defiles a person, but what comes out of the mouth that defiles him or her. Mark's writing includes the detail that Jesus and the disciples went into a house before having this conversation (Mark 7:17). This exchange does not take place within hearing of the people or the Pharisees.

The disciples ask Jesus if he knows that the Pharisees were offended by His comments. Clearly, the disciples are beginning to understand how significant the conflict between Jesus and the Pharisees is. That would have been uncomfortable for them. Pharisees were highly respected in ancient Jewish culture. They were seen as standards for righteous living and the final word on what was acceptable and what was not. The fact that the Pharisees were offended by the teaching of Jesus may have bothered some of the disciples. They cannot yet know that the offense of the Pharisees will eventually lead to their participation in Jesus' murder (Matthew 26:14–16; John 11:53).

It's not surprising the Pharisees took issue with what Jesus said. He was challenging the very basis of their teaching and rulemaking. Most faithful Israelites would likely have believed that putting the wrong food or unwashed hands in one's mouth was exactly what defiled a person before God. Jesus has just denied that idea, consistent with His prior explanations that a person's intents mean just as much as their actions (Matthew 5:21–22, 27–28).