Jesus is passing through a field of grain on the Sabbath and the hungry disciples pick and eat grains. The Pharisees are all over this like white on rice. (You can read it here.)
Here are a few notes that add to our understanding of the nuances of this passage.
The fourth controversy, like the second, involves a meal—but this time it is a meal on the go, the ancient equivalent of fast food. Mark notes several occasions when Jesus and his disciples are so busy ministering to the throngs of people that they have no time even to eat (3:20; 6:31; 8:1).
In drawing this comparison [between himself and David], Jesus is declaring that the requirements of his messianic mission (here, his disciples' need for nourishment on the road) take precedence over the prescriptions of the law. But he is also saying more than this. Jesus is likening himself to David, and his disciples to David's loyal band of soldiers. David was the "anointed one" who had been chosen by god to lead Israel (1 Sam 16:13), but who spent years being hunted down by Saul before finally taking up his royal throng. Like David, Jesus is the Lord's anointed one, his Messiah, pursued and persecuted by the leaders of Israel until the day when he will take up his throne. ...
George Montague, Catholic Commentary on Sacred Scripture: Gospel of Mark
"Son of man": the origin of the messianic meaning of this expression is to be found particularly in the prophecy in Dan 7:13, where Daniel, in a prophetic vision, contemplates 'one like a son of man' coming down on the clouds of heaven, who even goes right up to God's throne and is given dominion and glory and royal power over all peoples and nations. This expression appears 69 times in the Synoptic Gospels; Jesus prefers it to other ways of describing the Messiah -- such as Son of David, Messiah, etc. -- thereby avoiding the nationalistic overtones those expressions had in Jewish minds at the time.
when Abiathar was high priest: The priest who provided David with bread was actually Ahimelech, Abiathar's father (1 Sam 21:1). This apparent discrepancy causes some modern scholars to accuse Jesus of misquoting Scripture, although this conclusion is unnecessary.
Jesus probably mentioned Abiathar instead of Ahimelech to post a warning for the Pharisees. Abiathar is infamous in OT history as the last high priest of his line, who was banished from Jerusalem and the priesthood for opposing Solomon, the son of David and the heir of his kingdom (1 Kings 2:26-27). He thus represents the end of an old order that passes away with the coming of David's royal successor. As Jesus compares himself and the disciples with David and his men, he likewise draws the Pharisees into the story by casting them as figures like Abiathar. The Pharisees, then, represent an old order of covenant leadership that is about to expire, and if they persist in their opposition to Jesus, the new heir of the Davidic kingdom, they will meet the same disastrous fate that befell Abiathar. Jesus' allusion to this OT tradition was a subtle yet strategic way to caution the Pharisees against their antagonism to his ministry.
The Gospel of MarkThe Ignatius Catholic Study Bible