@owyn Depends on what edition they're playing. But more or less AC is the difficulty check that attack rolls have to beat (Outside of magic) in order to HIT.
I.E. AC (Armor class) = 22 So the enemy has to beat 22 with their attack roll
(Base attack bonus + modifiers) 6 + 1d20 = 22 =< # then they roll for damage and being 1 HP he's likely knocked unconscious, or if he's unlucky straight dead.
So in either case, he has 1 HP with a con modifier of -3, as a wizard who has at most a 4 HP roll he'll never be more then 20 hp without gear / buffs. So let's put it like this. A single punch on the hand is enough to K.O him.