Think back to your high-school bio and chem; remember the whole idea of unpaired atoms or groups if atoms running around? That's a free radical. Remember that

Think back to your high-school bio and chem; remember the whole idea of unpaired atoms or groups if atoms running around? That's a free radical. Remember that it takes energy to break chemical bonds? That's what UV does: provides the energy. The damage it causes is basically knocking random compounds and proteins apart. This leaves a random atom running around without an electron: a "free radical". Antioxidants donate an electron, thereby neutralizing the free radical. The real problem is when it does it to DNA; it can forcibly bond with it, and that's when you can get cancer. Which is why antioxidants are important.