Neutral Density stands for Neutral density. Using this A153 Neutral Grey filter, you can achieve a longer exposure time than would be allowed by the film speed or sensitivity of your camera's sensor. Think for example of capturing flowing water. This filter reduces light by approximately two stops.
Cokin A-Series
You will need an adapter ring and the Cokin A-Series Filter Holder to attach this filter to your lens. The "A Series" filters are recommended for use on lenses with a filter diameter between 36mm and 62mm, which have a focal length greater than 35mm.