What does Matthew 12:6 mean?

Pharisees have leveled accusations that Jesus' disciples have broken the Law of God. The men were plucking and eating grain on the Sabbath (Matthew 12:1–2). Jesus has responded with a series of questions (Matthew 12:3–5). Among these was the point that priests "break" the Sabbath—at least by the Pharisees' legalistic approach—when they work in the temple on those days. The Law allowed this because the commands of God for the temple must be obeyed above the commands to keep the Sabbath.

Jesus was showing that the Sabbath is not the most important command above all the other priorities of God. The work of the temple, for instance, had priority for the priests over the keeping of the Sabbath. While God's laws, as given, have meaning and authority, they are not meant to be applied with mindless literalism. Even if His disciples were breaking the Pharisee's concept of the Sabbath, they weren't violating God's intent for the Sabbath. In other words, the Sabbath rule, as intended by God, would not condemn what they did.

Now Jesus takes His argument in a new direction. Something has arrived that is even greater than the temple. Jesus is most likely referring to Himself as the Messiah. He understood that the Law and the Prophets pointed forward in time to the coming of the Messiah. He would fulfill the Law. He took priority even over the work of the temple, which itself took priority over the keeping of the Sabbath for the priests who did the work.

Jesus' point here seems to be that even if what the disciples did was literally against the normal meaning of the Sabbath law, they would still be guiltless because they were under the higher authority of the Messiah. This would apply in the same sense as a priest serving in the temple would be guiltless for their legitimate work.