Neutral density filters are either colorless or grey and reduce and/or modify the intensity of light without harming color rendition. This allows photographers to shoot in situations that typically wouldn’t allow them. These filters come in different intensities, which allow for different shooting techniques.
I recently purchased B+W ND 3.0 at B & H Photo for my Leica 21mm Elmarit lens. It allows me to shoot in very bright light, thus allowing proper exposure. It can also be used to create a time lapse picture which shows the motion of clouds or objects in front of it. As I was driving around I came across this old school (built in 1896) that appeared to be on its last leg. You can see in the pic, the motion of the clouds in this 4 second exposure.