A pass is permission to be away from one's Military for a limited period of time. Time away on a pass is not counted against leave, the annual allotment of days off from duty.
In the RAF, a "Permanent Pass" known as Form 557 would be issued to servicemen of good standing and six-months service allowing them to leave the quarters after duty until midnight and no further than eight miles (or as otherwise restricted on the form). It could be endorsed with permission to wear plain clothes.