Jamieson, Fausset and Brown on 1 Samuel 21:4

4. there is hallowed bread--There would be plenty of bread in his house; but there was no time to wait for it. "The hallowed bread" was the old shew-bread, which had been removed the previous day, and which was reserved for the use of the priests alone (Le 24:9). Before entertaining the idea that this bread could be lawfully given to David and his men, the high priest seems to have consulted the oracle (1Sa 22:10) as to the course to be followed in this emergency. A dispensation to use the hallowed bread was specially granted by God Himself.