Leviticus 24:9
And it shall be Aaron’s and his sons’; and they shall eat it in the holy place: for it is most holy unto him of the offerings of the LORD made by fire by a perpetual statute. And it shall be Aaron’s and his sons’; and they shall eat it in the holy place: for it is most holy unto him of the offerings of the LORD made by fire by a perpetual statute.
The consecrated loaves were set apart for Aaron and his sons to eat in a holy place, as a most holy share of the offerings presented to the Lord by fire under a lasting ordinance. Since eating was limited to the priests, partaking by anyone else was ordinarily unlawful, so David's taking of the bread together with his hungry companions fell outside the normal rule. That emergency is held up as precedent that urgent human need could make eating the consecrated bread guiltless. By that precedent, the disciples' rubbing out grain to sustain themselves when hungry was even more excusable.