The Warhammer world is a place of magic, of spellcrafted armour and magical protection, where creatures can have an arcane resilience at odds with a sometimes frail appearance.
Some models have a special save called a Ward save, typically as part of their special rules or from certain Magic Items or spells. The value of a ward save will always be shown in a model's entry in the relevant Warhammer Armies book. These work in the same way as armour saves, and may be combined with other Ward saves as normal. The key difference between ward saves and armour saves is that ward saves are never modified by the Strength of the attack. However, no model may have a Ward save better than 4+ by combining multiple Ward saves regardless of source; single Ward saves with a better value than 4+ are always specified as such.
Sometimes a model has both an armour save and a Ward save. Where this is the case, the model takes its armour save as normal. If the armour save is failed (or modified to the point at which the model cannot pass it) then the model takes its Ward save.