A neutral density filter is used to reduce the amount of light entering the lens, the purpose of this allows a photographer to adjust the aperture, shutter speed and ISO which without the (NDF) would create an over exposed image, what can be achieved by doing this is a shallower depth of field or motion blur.
Using a neutral density filter can also be used to show under expose natural lighting, which creates more tones in a photograph. this is an example of that.
Subject and Sequence 17076811 