AAU stands for Alpha Acid Unit. It is determined by the alpha acid % of the hop and the weight of the addition. It is better explained by John Palmer here
So, I'm making an Imperial IPA and it calls for 20-25 AAU's for bittering hops of like Columbus, Warrior or Summit. If I take the Summit AAu of 18.5% and take the high end of the AAU's of 25 for the recipe, I come up with about 0.75 oz of hops for that boil of 60 min. Does that sound right?