What does Matthew 12:8 mean?

Jesus concludes His rebuke of the Pharisees for their false accusation against His disciples (Matthew 12:1–2). The capstone of His response is a simple declaration.

Jesus uses the phrase "Son of Man" throughout Matthew to describe Himself. His biggest problem with the Pharisees is they have failed to recognize Christ is the Messiah. This is something they refuse to acknowledge, despite witnessing so many of His miracles and hearing His teaching (John 5:39–40).

In this specific case, Jesus' critics have attempted to condemn His disciples, supposedly for breaking the command to keep the Sabbath (Exodus 20:8–11). The conflict, however, is really with the Pharisees' own false interpretation of the laws. This confusion would not have happened, had they understood Jesus to be the Son of God. God, of course, is Lord of all. He also holds priority over the Law. Jesus is saying, then, that in the end He is Lord even of the Sabbath.

It's important to see that Jesus is not giving Himself a blank check to break the Law of Moses. Jesus fulfilled the Law by keeping it perfectly. He is, however, declaring that He made the Sabbath and is the final judge of what it means to keep the Sabbath or not. He declares His disciples guiltless from breaking the Sabbath requirement by their simple act of plucking and eating grain.