ND is neutral density, or perhaps "grey" filter. It makes your shot darker.


Folks often become familiar with a GND, or graduated neutral density, filter first. It's usually square, tinted grey on one half and clear on the other. It's slid in front of your lens, and often used to darken skies to balance bright skies against dark foreground.


ND filters were common in film days, when you might have high-speed film in the camera but need the aperture/shutter performance of lower-speed film.


I've wished for some ND filters in my bag at times when using a fast lens (85/1.2) outside and wanted thin DoF but slow shutter speeds. Even on ISO "50", f/1.2 was resulting in speeds of 1/2500 or a little faster. Had I used a 5-stop ND (if such an animal exists), I'd have shutter speeds of 1/60th or so, creating some motion blur in my shot but keeping the thin DoF.