What does Matthew 15:9 mean?

In response to criticism that He does not honor their manmade traditions, Jesus has boldly declared a group of scribes and Pharisees as hypocrites. This would not have been the reaction they expected when they came from Jerusalem to confront Jesus about the behavior of His disciples (Matthew 15:4–8).

Jesus is quoting a passage they likely know quite well: Isaiah 29:13. It contains words the Lord said to Isaiah about the Israelites of that generation. This exchange with Jesus reveals that the words of Isaiah's prophecy apply to Israel's current religious leaders, as well. The hearts of the Israelites in Isaiah's day were far from God, despite honoring the Lord with their words.

Now Jesus completes the quote by declaring that their worship of God is useless: it is ineffective and pointless. This is because they are teaching their own preferences, their traditions, as if they were the doctrines given by God.

This accusation from Jesus is impossible to dodge: the last part of the quote from Isaiah is exactly why Pharisees had come. They were attacking Jesus and His followers for ignoring their traditional, manmade, ritual version of handwashing before meals. This was never commanded by God in Scripture. It was one of the "commandments of men," and Jesus treated it that way, revealing the Pharisees' hypocrisy.