Free radicals are atoms or groups of atoms that can be very dangerous to the human body.
Free radicals occur when various types of atoms interact with oxygen and thereby come to have an odd number of electrons.
If free radicals interact with certain important components of cells, the interaction can lead to many sorts of negative effects on human health. These include, most notably, cancer.
It is because of these potential harmful effects of free radicals that it is important for people to consume antioxidants. Antioxidants can rid the body of the free radicals.