One day, Jesus and his disciples are plucking heads of grain to eat as they were walking through a field on the sabbath. The Pharisees get on Jesus' case about it, telling him, Look, your disciples are doing what is not lawful to do on the sabbath (Mt 12.2). Jesus' response is effectively to point to other occasions in the Old Testament in which persons apparently break sabbath without incurring any guilt, he rebukes the Pharisees for not knowing the scriptures and for condemning the guiltless, and he ends with this fantastic line: For the Son of Man is lord of the sabbath (v. 8).
What is so powerful about that line? What does it mean? It's meaning and significance are in fact tremendous, because effectively Jesus is offering himself as the definitive interpreter of God's moral law. He is the one who will tell us with authority whether we have rightly understood what God was getting at in giving us the law, or not. And what is so special about that? Because he is claiming special, privileged access to the mind of God, beyond what any mere moral teacher or rabbi could know.
To put it another way, we might say that in this manner, Christ presents himself as the logos or wisdom of God. As the gospel of John says, the Logos which was in the beginning with God, and which was God, has come to make God known (John 1.1, 18). This understanding of Christ as logos is explicit in John. But it is also implicit in Matthew, because that is evidently the way Jesus presented himself during his time on earth.
The Son of Man is lord over the sabbath. That means that Jesus Christ teaches us the proper understanding of God's righteousness, because we may say that Christ is the mind or wisdom or logos of God come down to earth, united with human nature, so as to communicate the truth to us. Anyone who loves God and wishes to know him, therefore, ought to consult with Jesus.