A polish is a substance used to give vehicles a smooth shiny surface and to eliminate scratches and dirt when rubbed in. Polishes contain abrasive elements that scrub the paint surface, actually removing a thin layer of paint from the car. The polish also contains solid components that settle into fine scratches and minimize the appearance of the scratches. They do this by reflecting light and making them blend into the car's surface.
Most manufacturers suggest that you apply several coats, each with different degrees of pressure. This will help to gradually minimize the appearance of imperfections in the car's surface.