Deuteronomy 23:25
When thou comest into the standing corn of thy neighbour, then thou mayest pluck the ears with thine hand; but thou shalt not move a sickle unto thy neighbour’s standing corn. When thou comest into the standing corn of thy neighbour, then thou mayest pluck the ears with thine hand; but thou shalt not move a sickle unto thy neighbour’s standing corn.
When coming into a neighbor's standing grain ready for the sickle, a hungry person may pluck ears with the hand and eat enough to satisfy immediate hunger, rubbing the kernels free in the hands. This permission covers a traveler entering the field and eating by hand what is needed for the moment. It does not permit putting a sickle to the neighbor's standing grain, cutting the standing crop, or carrying it away as a harvest. It grants food for immediate need, not a right to harvest another person's crop. In this way the poor and hungry are given access to food while the landowner's crop is protected from being appropriated.
As they passed through a grainfield on the Sabbath, the disciples picked heads and rubbed them to eat, acting under this permission to relieve hunger. They were hungry after long work of teaching and healing, and they ate to satisfy hunger and sustain bodily strength for use in the service of God. Their gathering and rubbing the grain to eat was permitted on ordinary days. Religious leaders treated the picking as reaping and the rubbing as threshing, and so condemned the act when done on the Sabbath as a violation of their Sabbath requirements.
Such picking and eating to meet the body's need remains right and lawful even on the Sabbath. The Sabbath was made to give rest and peace and remind of the Creator, not to interfere with the needs of the body. As Lord of the Sabbath, Jesus acquits those who do this from blame under the very statutes they are accused of violating.