A pavise is a large shield which each Crossbowman can prop up in front of him. To represent this, Crossbowmen equipped with pavises have an additional +2 Armour save versus normal and magical missile attacks. They do not get this bonus in close combat. So a Crossbowman in heavy armour with a pavise gets an Armour save of 3+ versus missile attacks, but only an Armour save of 5+ in close combat.