Some troop types and creatures are protected by more than mere physical armour. They may be shielded by magical charms, preserved by the gods, or perhaps are just astoundingly lucky.
Models with this sort of protection are referred to as having a ward save or ward. This type of save is quite different from an armour save and it is very important to understand the difference. Wards represent magical or divine protection that can save a warrior when armour would be of no use at all. Unlike an armour save, a ward save is never modified by Strength modifiers, etc. Even if a hit ignores all armour saves, a model with a ward may still try to make its ward save as normal.
Sometimes, a model has both an armour save and a ward save. In this case, the model must take the armour save first and, if it is failed, the model is allowed to try to make a ward save. No model can ever try to make more than one ward save against a wound it has suffered. If a model has two ward saves for any reason, use the better ward save. Note that, regardless of any modifier, a roll of 1 is always a failed save nobody is invulnerable!