Your armor class (or AC) is a number that describes the amount and the quality of armor being worn. Armor class will generally run from about 0 to 200, though exceptionally good armor can improve even on the latter figure.
The larger your armor class, the more protective it is. A negative armor class would actually help get you hit. Armor protects you in three manners. First, it makes you harder to be hit for damage. A hit for no damage counts as a miss, and is described as a miss. Second, good armor will absorb some of the damage that your character would have taken from normal attacks. Third, acid damage is reduced by wearing body armor (but the armor may be damaged instead). It is obvious that a high armor class is vital for surviving the deeper levels of Angband.
Each piece of armour has a base armor value, which, like the damage from weapons, is assumed known by the player, and a magic bonus, which will not be displayed unless the armor has been identified or was bought in a store.
Armor class values are always displayed between a set of square brackets, as '[#]' or '[#,+#]'. The first value is the base armor class of the armor. The second number is the magical bonus of the item, which is only displayed if known, and will always have a sign preceding the value. These plusses can be determined by wielding the armor in combat and being hit. Note that a few rings, amulets, and weapons also have the '[+#]' notation, indicating that they provide an armor bonus. Many pieces of heavy body armor will also have a '(-#)' (in normal brackets) before the '[#,+#]', which indicates that the weight of the armor decreases your chances of hitting monsters. This can range from nonexistent for very light armor to '(-8)' for the heaviest armor!