You are less likely to be hit in combat if you have this Perk. You gain a +5 to your Armor Class, in addition to the AC bonus from any armor worn.”— Fallout 2 description
+5 to Armor Class per rank, stacking with other bonuses. There are two ranks in Fallout and Lionheart and just one in Fallout 2 and Fallout Tactics. It's a decent option whenever a better perk is not available.