The term mirror polishing means the process implemented to make sure that the treated surfaces shine of reflected light.
This metalworking process is a preparatory operation to other kind of treatments such as chromium plating, gilding, silvering, but in the case of stainless steel it can be a final solution.
In the real world there are not perfectly smooth surface. They all have a more or less high degree of surface roughness due to microscopic roughness of metal, which disperse the light rays in multiple directions.
The aim of the polishing process is precisely to shape these microscopic irregularities in order to make the treated surface as planar as possible, so that the reflection of light became more uniform, giving to the surface the typical shiny appearance.
This result is achieved by controlled abrasion of the surface, using very fine-grain abrasives that effectively reduce the magnitude of the surface roughness.