The claim to fame of vitamin E is as an antioxidant. In other words, it is a scavenger of free radicals such as reactive oxygen species (e.g. superoxide, hydrogen peroxide). Free radicals are generated by numerous processes within cells and have the ability to damage cell membranes, proteins and nucleic acids. Vitamin E is at the forefront of defense systems to prevent oxidative damage, and due to its lipid solubility, is particularly important in protecting cell membranes.
Mechanistically, when vitamin E absorbs a free radical, it is converted into a radical itself. The resulting tocopheroxyl radical is then reduced back to tocopherol by glutathione, vitamin C or other molecules.