Antioxidants mitigate against wear and tear in the body from oxidative damage from "free radicals". Free radicals are highly reactive chemicals that react with and can damage tissues within the body, mainly because of the oxygen-based reactions, or oxidation. For example, oxidative damage to a cell's DNA may cause them to divide uncontrollably, while similar reactions to cholesterol particles in the blood can irritate the lining of arteries.
Antioxidants are the body's natural protection, some of them generated within the body, but these can be supplemented by antioxidants in our diets. Good examples are chorophyll in plants, lycopene colours in tomatoes and vitamin C. In fact, the best sources of antioxidants are to eat decent levels of fruits and vegetables.