The Sabbath is the seventh day of the week, Friday sundown to Saturday sundown, on which the Torah forbids work (Exodus 20:8–11). By Jesus' day the oral tradition had worked that command out into detailed categories of forbidden labor, later written down in the Mishnah (the first great code of Jewish oral law, compiled about AD 200) as thirty-nine categories, reaping and threshing among them (Mishnah Shabbat 7:2). Observant Jews keep these categories to this day.

So the disciples' act reads differently on each side. Plucking a neighbor's standing grain by hand was expressly permitted by the Law itself (Deuteronomy 23:25); this was never about stealing. But in the tradition's categories, plucking counted as reaping, and rubbing the heads as threshing: work, on the day work is forbidden. That is the collision in the scene, and it is why Jesus answers from Scripture's own precedents (David and the showbread, the priests in the Temple) and from who He is: “the Son of Man is Lord of the Sabbath.”