Antioxidants are added to foods containing fats/oil. Why?

An antioxidant is a chemical that prevents oxidation. Oils are particularly prone to oxidation and are said to become 'rancid' when they do. Most of us find the odor of 'rancid' oil bad(or worse) and therefore it is considered undesirable. Consequently, it is quite common to add antioxidant to oils(oily food).

Similarly, fats can also undergo oxidation which can make them unpalatable. Thus antioxidants are added in them.