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.
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.