this is a gray filter, used in photography and filmography when you have too much light for your purposes.
It is important that a neutral density filter (also abbrevietad ND) be truly neutral, that's to say, that it really absorb equally all the visible wavelengths.
One situation where you want a ND filter is, for example, if you need long exposure times for some application, but ambient light is too intense even if you stop down all the way (which produces problems with diffraction, BTW). If this is your case, remember that you will still have reciprocity failure.
This filter is not widely used: his friend, the graduated neutral density filter, is much more common.