Neutral Density (ND) filters reduce the intensity of light entering the lens and modify all wavelengths equally so there should be no change in rendition of color. They therefore allow larger apertures and or longer shutter speeds to be used when they would otherwise result in over exposure, allowing more creative control over depth of field and recording motion in moving subjects. They are available at various strengths indicated by the number after the ND:

ND 2 (0.3) absorbs 1 stop of light
ND 4 (0.6) absorbs 2 stops of light
ND 8 (0.9) absorbs 3 stops of light
ND 16 (1.2) absorbs 4 stops of light

Example use: With the maximum shutter speed of 1/500 it’s impossible to achieve a shallow depth of field in a scene lit by daylight. Using ISO 200 film the biggest aperture you could use would be f/11 at 1/500. If you were to add an ND2 you could shoot at f/8, ND4 f/5.6, and an ND8 filter you could open up to f/4. Likewise, instead of increasing aperture you could increase the shutter speed if you wanted to blur subjects in motion.