Free radicals are molecules that contain oxygen and are missing one electron. This makes them very aggressive and reactive. They snatch electrons from other molecules and thus also convert them into free radicals, which is equivalent to a chain reaction. Free radicals disturb important metabolic processes and can damage important proteins, e.g. in the cell membrane, as well as the genetic material of the cell (DNA). They thus promote tissue damage, functional disorders or even the development of tumours.
See also: Antioxidants