Oxidation reactions occur when the combustion of oxygen that keeps us alive and well produces byproducts called oxygen free radicals. When this process occurs in metals, we call it rusting.
Free radicals are molecules that have lost an electron.
When this happens to oxygen, we call it singlet oxygen because it has only one of its electrons left. This is a highly unstable condition. If DNA is involved, mutations occur - a favored theory of a common cause of cancer. If low density lipids are involved, atherosclerosis ensues. Over time, free radical damage accumulates in all our tissues to one extent or another, thereby aging us.