They react very easily with other molecules, and can damage cells. High levels of free radicals seem to cause a lot of the damage associated with aging. Free radicals are produced as part of normal body chemistry. Antioxidants are molecules that can stop free radicals from reacting with other molecules. This limits the damage they do.
Several nutrients are antioxidants.
Antioxidants are important for people with HIV, because HIV infection leads to higher levels of free radicals. Higher levels of antioxidants can slow down the virus and help repair some of the damage it does. Unfortunately, it's not that easy. Instead, they are the minimum amounts needed to prevent shortages in healthy people. HIV disease and many AIDS medications can use up some nutrients.