Neutral Density filters are the most commonly used along with C-PL and UV filters. These filters, instead of the ones that protect the lens from damage or UV pollution, were created to decrease the volume of the light entering the lens. True ND filters decrease light equally to achieve the same density through the all visual spectrum giving no changes in color balance of the image. That is why the called Neutral Density.
ND filter decreases the amount of light entering the camera lens. This allows to shoot with wide open diaphragm even in shiny day, to add motion blur to subjects and to extend time exposures.