Armor Piercing Protection is a gear perk that improves your defense against armor-piercing rounds. It helps a lot against FND soldiers who carry that ammo and punch through cover.
You take less damage from armor-piercing bullets. Many tougher FND soldiers use them, so the effect shows up in most big fights.
Wear one of the gear pieces that carry it: Rioter Helmet, Rioter Pants, Rioter Vest. Each piece only takes one body slot, so plan around the other perks you want.
Worth it if armor-piercing troops keep killing you. If you can get Armor Piercing Defender on the same slot, that is the stronger choice.