As i understand, every weapon or shield has 3 categories:
BLOCK POWER - This is the base amount of damage you will negate while blocking normally (modified by your blocking skill)
PARRY FORCE - This determines how far enemies will be knocked back when parried
PARRY BONUS - modifies BLOCK POWER by PARRY BONUS when successfully parried attack.

So if you have 10 BLOCK POWER on your weapon and x4 PARRY BONUS, successfully parrying will negate 40 dmg before armor and resistances.