The best defence against free radicals
Antioxidants can be found in many plants such as green tea and carrots. Their primary role is to combat excessive "free radicals", highly unstable molecules that tend to attack our cell components if left unchecked. These free radicals are naturally produced during normal cellular respiration, but certain external factors (pollution, cigarette smoke, the sun's UV rays, etc.) can increase their production, so that it becomes impossible to control their impact.
This is why our body needs the help of "antioxidants", natural molecules, usually derived from plants, whose role is to slow down the oxidation created by excessive amounts of free radicals. By attaching themselves to free radicals, antioxidants cause a new chemical reaction which blocks their action.
The problem is that each antioxidant can only react with a single free radical, which means that our body needs to continually replenish its stock of antioxidants. It therefore extracts them from food, which contains the essential nutrients the body needs in order to function effectively.