Some weapons or creatures are so powerful that they can punch right through armour. Such attacks inflict a modifier on the saving throw, just as shooting modifiers affect the To Hit roll.
An attack of Strength 4 inflicts a save modifier of -1, with the modifier growing a point higher for each additional point of Strength. Note that this means a model could be hit by an attack whose armour save modifier makes the armour save impossible to pass – in which case, the save is automatically failed.
For example, a model with a 3+ armour save is hit by a Strength 5 Attack with a -2 modifier. This means the model now only has a 5+ save left to try and save the Wound.