Neutral Density stands for Neutral density. Using this X153 Neutral Grey ND4 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 running water. This filter reduces light by approximately one stop.
Cokin X-PRO Series
The X-PRO series is aimed at professional photographers. You need an adapter ring and the Cokin X-PRO Filter Holder to attach this filter to your lens. The "X-PRO Series" filters are recommended for use on lenses with a filter diameter between 62mm and 112mm.