Rating from -10 to 10, with the highest number being the lowest resistance to damage. AC of 10 is like being naked, while -10 is armored like a tank, or a high level ninja.
A ninja does not need any armor protection, he can go in naked and still reach -10 AC when leveled high enough.
Armors, shields, etc... with +1 +2 +3 add more protection by removing (lowering) points to the AC.
Some items have +? modifiers and are cursed, and actually can remove AC protection instead of adding.