â€œNeutral Density filter are, in essence, just grey filters used to reduce the amount of light passing through the lens and hitting your cameraâ€™s sensor/film. So, in short, we use them in photography the same way you would wear sunglasses in real life!â€ writes Antoine Khater, there at adidap.com.
They are said to be neutral because they do not affect the colors in the resulting picture; they filter out all colors equally, adds Antoine.
You need to use an ND filter if you want a shallower depth of field in a shot that might limit your maximum shutter speed. An ND filter will allow you to shoot at larger apertures.
The ND filter can also be useful if you want to shoot at a slower shutter speed such as shooting a brook or a stream. A slower shutter will capture that milky water effect without overexposing your image because the ND filter will eat up that extra light.