Parry is the rating at which you will avoid any incoming melee damage. The defender's chance to parry is increased by his Weapon Skill, while the attacker's chance to ignore parry is increased by his Strength. This means, that the higher your weapon skill, the higher your chance to parry is, and the higher your opponent's strength, the higher the chance of your parry being avoided is.
Parry gets increased by 10% when wearing two one-handed melee-weapons, also known as dual wield. Example of such classes are the Disciples of Khaine, Witch Elves, Choppas, Slayers and Marauders. This modifier, as well as all other percentage modifiers, are never affected by strength.
The chance to parry is calculated in the following way:
<chance to parry> = (<weaponskill>/<strength>*0.075)+(<defenders increase in parry> - <attackers reduction in parry>