After a break of a couple of chapters, the Pharisees are back attacking Jesus. Remember that the Pharisees were legalists, not just demanding obedience to the 10 commandments, but to 613 rules mostly made up by men. One of the rules had to do with washing of hands before the meal. According to the Pharisees, it was a SIN to eat with unwashed hands, not simply a matter of bad hygiene.
Jesus confronts them with a passage out of Isaiah. The problem with legalism is that religion is no longer a based upon that faith relationship with God, but it becomes about obeying the rules. The more the focus is placed on the rules, the less and less it is focused on our relationship with our Living God.
Who is this Jesus? One who wants us to have a relationship with us based upon our trust in Him, and not our saying the right words or doing the right things.