An Armor-Piercing (or "AP") hit ignores half of whichever Protection stat it goes against. This is before the DRN Damage-vs.-Protection roll, which determines how much the taken Damage varies from the typical DMG-minus-PROT equation.

Acid Attacks, Cold Attacks, and Fire Attacks are always at least Armor-Piercing.

Piercing Attacks innately have a lesser sort of Armor-Piercing, ignoring 15% of whichever Protection stat they go against. There are Armor-Piercing Piercing Attacks, though, which ignore 65% – Crossbow bolts, for example.