Antioxidants either block the initiation of free radical (ROS, reactive oxygen species) formation or inactivate (e.g., scavenge) free radicals and terminate radical damage.
Examples are the lipid-soluble vitamins E and A as well as ascorbic acid (vitamin C) and glutathione in the cytosol.
References
Finkel T, Holbrook NJ: Oxidants, oxidative stress, and the biology of ageing. Nature 408:239, 2000.