The main theme of this chapter in Mark is that Jesus is the son of God and that his divinity outweighs the law and Jewish tradition. In the chapter, we see multiple instances of Jesus doing things that have the "teachers of the law" or the Pharisees upset. He and his men pick grain on the Sabbath. He forgives the sins of the paralyzed man and goes and has dinner with Levi the tax collector.
In each case, Jesus responds to criticism essentially by saying that he is the "Son of Man." He says that he is above the law and that they need to understand that. The chapter centers around Jesus's statements about who he is and how the laws and traditions do not apply any longer.