A clear coat is the top coat of a paint that contains no pigmentation or color. In a typical automotive paint surface, there is a primer coat which is what is sprayed on the metal or plastic body surface, a base coat, which contains the color and a clear coat. A clear coats purpose is to provide depth and gloss to the finish (known as DOI, or Distinction of Image in the industry) and to protect the base coat from the elements, including the harmful UV (Ultra-violet) rays of the sun.
Clear coats became popular in the early 1990s and currently over 90% of new cars feature a clear coat as the top (exposed) surface.
Clear coats require special consideration. Because they are clear, scratches and swirls tend to become more prominent since the clear characteristic causes such defects to be magnified. For this reason, avoiding scratches and swirls is critical if you want your clear coated surface to look good.
Most detailing products available today are clear coat safe and are so labeled. This generally means that they are non abrasive or minimally abrasive.
The best and easiest way to determine if you are dealing with a clear coat is to check the owners manual of the car. If that is not an option, apply a small amount of polish to a rag that is not the same color as the car and apply it to a not normally seen paint surface. If the color does not appear in the cloth surface, then you have a clear coat.
The protection provided by a clear coat will not prevent oxidation but it does help the situation considerably. The clear coat itself can lose it's oils which makes it vulnerable to oxidation, turning the surface dull.
Although a clear coats protective properties make modern paint jobs last longer, they still have to be maintained with oils (applied as part of a polish job) and a protective wax. Unmaintained, a clear coat can disappear, leaving the base coat exposed to the elements something that it was not designed for. If your clear coat is gone, a new paint job is required.