What does Mark 2:25 mean?The Pharisees are accusing Jesus' disciples of dishonoring the Sabbath by plucking grain and eating it. According to their traditional laws, this was an act of "work" which was not allowed on that particular day. Instead of defending them by arguing about the law, Jesus starts with the heart-attitudes of the Pharisees. Particularly, their potential hypocrisy in defending someone like David who, several hundred years prior, did something related to ceremonial laws which was arguably far "worse."
By this point, the Pharisees' interaction with Jesus has changed from wonder (Mark 1:27) to silent criticism (Mark 2:6–7) to trying to sow doubt in His disciples (Mark 2:16). Now, they directly accuse Jesus of endorsing His followers while they break a very serious law.
Jesus' responses have always attempted to get His accusers to think—about the Law, the needs of the people around them, and His identity. Jesus uses the Pharisees' own beliefs to show how their thinking has become twisted.
First Samuel 21:1–9 tells the story of a starving David trying to get food while evading King Saul's notice. He sneaks into the tabernacle and convinces the priest to give five loaves of showbread to him and his men. Also called "the bread of the Presence," showbread is twelve loaves of bread that sit on a table in the tabernacle for a week, after which they are eaten by the priests.
By the literal letter of the law, David should not have eaten the showbread. In a somewhat related sense, the disciples should not have plucked and sorted the grain, at least according to the Pharisees' traditions. But, there is meant to be a higher authority than the letter of the law.