1 Samuel 21:4
And the priest answered David, and said, There is no common bread under mine hand, but there is hallowed bread; if the young men have kept themselves at least from women. And the priest answered David, and said, There is no common bread under mine hand, but there is hallowed bread; if the young men have kept themselves at least from women.
Ahimelech had no ordinary bread under his hand to give, only hallowed bread to offer. He would give that hallowed bread only if the young men had kept themselves at least from women. David, hungry with his companions, overcame the priest's scruples, received the consecrated bread, ate it and shared it with them to satisfy hunger.
Although the shewbread was ordinarily reserved for priests, extreme and urgent hunger excused its use, so the act was guiltless. This was especially so to sustain those engaged in God's service. Christ appealed to this precedent to show that meeting human need is right, so His hungry disciples were not to be condemned for plucking grain on the Sabbath.
The episode shows that exceptional need can justify using what is ordinarily set apart. It does not excuse careless neglect of the tithe.