The piercing property is identical in function to the Amazon skill Pierce; it gives a projectile a chance to pass through one target and continue on to hit a second (or third, or fourth, or fift) for full damage. Bonuses to Piercing stack with the Amazon's Pierce skill.
Numerous unique bows/xbows have 100% Piercing as a property, and you can find +33% piercing on the Unique Razortail Belt as well as several other items.