Weapon Skill (WS) determines a character's ability to fight in hand-to-hand combat, reflecting natural aggression, disposition, and training.
It is expressed in 'percentage' terms; i.e., from 01 to 100%. The higher the WS, the easier it is to strike a blow in hand-to-hand combat or to parry an opponent's blow.
For tests, WS reflects hand-to-hand combat ability; it might be used in non-combat situations when a character is using something like an axe or sledgehammer. Remember that a target that isn't moving counts as prone.