Armor Class

Armor Class is a game statistic which determines the chance that a physical attack with melee or ranged weapons can avoid a hit, it does not reduce the amount of damage dealt in the case of a successful hit. This number is between 20 and -20 and the lower your number, the better the odds are that the creature can avoid a lot of blows before getting hit.

Dexterity and armor
Dexterity gives any character you create a bonus or a penalty depending on the number. body armor and shields also influence armor class and heavier armor gives you a better armor class, but the heavier and more protective armor can only be worn by members of the warrior group.

Armor class in the game files of Baldur's Gate and Baldur's Gate II
If you look in the games, you have column Parameter 1, which is the number and parameter 2 which determine the type of armor class. There six types of armor class.