A spawn point is a point at which a mob that has been killed will respawn after its respawn time is up. Some mobs wander around a bit; the spawn point serves as sort of a "home" or "center" around which to wander. Mobs that have aggroed on someone will leash back to their spawn points once they have nobody to fight (i.e. once their aggro list is empty), whether that is because they killed their target(s) or because their target(s) got away. Patrolling mobs will leash back to the spot they were patrolling just prior to being aggroed.