We live in an oxygen rich environment and thus our cells are exposed to oxidative stress, generally also called ageing. Oxygen wants not only to react with our food (fuels) to provide energy, but all body parts are exposed (skin, eyes, blood, organs). Adverse reactions of free radicals (free electrons that like to ‘attack’ other bio chemicals and create havoc) are to be prevented and for this anti-oxidants are needed. We take these from our diet. There is a whole range of anti-oxidants, the specific roles of which are not so very clear yet. In general they catch free radicals, prevent oxidation and reduce the risk of getting cancer. Vit C and E are also anti-oxidants.
Nature makes these antioxidants also to prevent stress happen to plants and so we can find antioxidants in the leafy vegetables and also in tea, grapes and certain herbs. There is a whole set of natural chemicals occurring in plants called phytonutrients, among which are anti-oxidants and sterols) which have benefits for health.
Nuts and seeds and the cold pressed, untreated vegetable oils derived from these also contain several anti-oxidants.