This 72 mm X Series ND Filter from Zuma allows you to maintain a wider aperture by reducing the light reaching the sensor. This ND filter has 16 layers of multicoating to prevent unwanted reflections.
A neutral density filter (as the name implies) is a filter which is color neutral, but reduces light transmission.
In photography. the amount of light which reaches the sensor can be controlled by the ISO sensitivity, the aperture (f/stop) of the lens, and the shutter speed. There may be times when a necessary effect can be achieved only by using a lens at the wider apertures (for out of focus backgrounds for example.)