Poison Protection is a gear perk that cuts the damage you take from poison gas and poison rounds. It comes on the Hazmat pieces, so it matters most when you fight FND troops who use poison weapons.
Poison clouds from grenades, mines and poison ammo hurt you less and for less time. It does nothing against bullets, fire or explosions, so it is a narrow pick.
Equip the Hazmat Gloves, Hazmat Pants or Hazmat Suit. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Keep it for areas where enemies throw a lot of poison, or when you use poison gadgets yourself at close range. For general play a broader defense perk does more.