Oxidation, (Redox (reduction–oxidation)) is a process in which a chemical substance degrades in the presence of oxygen/ozone or other oxidizing agents. When this happens to metals, we call it 'rust', however it will also happen to plastics, rubbers and paintwork.
When it occurs on paintwork, the surface of the paint will become dull and may appear hazy, milky and flat, on very oxidized paintwork, it may even appear powdery. This 'dead paint' is fairly easy to remove with even hand polishing, as oxidized paint is usually soft.
It can be prevented by coating your paintwork with wax, which seals the paint against oxygen and will become oxidized instead (a sacrificial layer). Speciality coatings such as ceramic are more resistant to oxidization.