Schneider Optics Neutral Density Filters (ND or N.D.) reduce the amount of light passing through the camera lens without changing the color of the scene. Especially useful in bright light conditions to help prevent over exposure. Allows proper exposure at wider lens openings, for reduced depth-of-field, and to highlight a key subject by making foreground and/or background out of focus. Permits slower shutter speeds for blurred motion effects in still photography.
An ND 0.6 (aka ND6) allows you to open the camera aperture two full stops OR, on motion picture cameras you can reduce the frame rate by cutting it in half twice (24fps to 12 fps to 6fps). For still photography you can reduce the shutter speed by half twice (1/250th to 1/125th to 1/64th)