If you are attacking with a weapon which has the Armor piercing property you may class their Armor Class (AC) as a number equal to the parenthesis following lower if they are wearing non-magical armor(Natural armor and spells such as Mage Armor do not count).
For example, if a bandit with Leather Armor and an Armor Class (AC) of 12 is your target and you have Armor Piercing(2), you may treat their Armor Class (AC) as if it was 10 instead of 12.