In Luke 6:1-5, Jesus' disciples were walking through a field and rubbed some grain together because they were hungry. Simple enough. Catch is, it was on a sabbath day and the Pharisees were nearby and caught this outrageous act. The Pharisees had extrapolated from the general command forbidding work on the sabbath that this action (clearly denoted in their works as reaping and threshing) was a sin. And they demanded of Jesus to know why they were doing this while He did nothing about it.
Jesus reminded the Pharisees that on another occasion, noted in sacred Scripture, David and his men were hungry they entered the Lord's house and ate the consecrated bread (also called Show Bread or Bread of Presence). This item is described in Exodus 25:30. There were twelve loaves of bread placed each week in the Holy Place, on a table that was overlaid with pure gold. Every Sabbath the 12 loaves were replaced with fresh ones, the priests ate the old ones. The account is in I Samuel 21:1-6 and 22:9,10.
Jesus' point was this. If David had the right to ignore this ceremonial provision from God, shouldn't Jesus, the Son of God and David's antitype have the right to set aside a ceremonial law in their time of need? This was a constant theme for Jesus throughout the Gospels, engaging the Pharisees in their own distorted view of God's law. The Phrisees could not see beyond the letter of the law and even added to it. Jesus wanted them to see that He is Lord of the sabbath.