AC — Armor Class
How difficult a creature is to hit. Higher is better: an attack hits when its total is equal to or higher than this number.
Armor Class (AC) is a number representing how difficult a creature is to successfully hit. Armor, shields, agility and cover can all improve it. When someone attacks, their attack total is compared with the target's Armor Class to decide hit or miss. Exactly how they are compared depends on the ruleset.
In Fantasy RPG
Fantasy RPG uses ascending Armor Class: higher numbers are harder to hit. An attack hits when the attack total (the d20 plus the attack bonus) is equal to or higher than the target's AC. Without armor a character has 10 + their Dexterity modifier. Armor the class is trained to wear replaces that, and a shield adds to it. Cover adds +2 (partial) or +5 (cover) for that attack.