Antioxidants reduce the damaging effects of oxidization in the body's cells. They're nutrients (vitamins and minerals) as well as enzymes that play a role in fighting off free radicals in our bodies that can cause us to become ill. Using antioxidant supplements will improve your body's ability to fight off infection and disease.
This is especially important for athletes who put their bodies under a huge amount of stress when training, and also for those who are on restrictive diets that don't allow them to eat at least five servings of different fruits and vegetables each day.