A dungeons and dragons term for Armour class.

It's how you determine if an attack hits you.

Say you have an AC of 15 and an attacker tries to hit you they roll a 15 or higher with a 20 sided dice it hits you. (Meets it beats it)

However your opponent also can have boosts to to their attack say a magic weapon adds plus 1 or something so they could roll a 14 plus the 1 to reach 15 thus hitting you. Then they roll damage with another dice dependant on the weapon.

So if the characters are then married they get a plus two to their AC and now have 17 and the 15 misses