While fleeing from Saul (1 Samuel 21), David and his companions went to Nob, a place where the tabernacle was located. The priest Ahimelech gave them the consecrated Bread of the Presence (special bread set before God) that only priests were allowed to eat (Exodus 25:30; Leviticus 24:9).
Jesus meant that meeting human needs is more important than strictly following the law. There may also be a comparison between David and Jesus, since both were chosen and anointed as kings of Israel.