Neutral Density stands for Neutral density. Using this Z153 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 two stops.
Cokin Z-Pro Series
The Z-Pro filters are available in two sizes. The more "normal" filters have a size of 100 x 100mm. The filters with a gradient have a size of 100 x 150mm. All of these filters are made from high quality CR39 organic glass. You will need an adapter ring and the Cokin Filter Holder Z-BZ100 to attach this filter to your lens.