An ND filter is sunglasses for a lens. It reduces light uniformly across the visible spectrum — the neutral part — so exposure drops without any color cast. Strengths are quoted in stops: a 3-stop ND cuts light to an eighth, a 10-stop to roughly one-thousandth.
It solves problems that aperture and ISO cannot. In daylight, shooting wide open at f/1.4 for shallow depth of field can demand a shutter faster than the camera offers. For video the constraint is stricter still, because the 180-degree rule pins shutter speed to frame rate, leaving ND as the only way to control exposure without changing the look.
It also enables long exposures in daylight: smoothing water into mist, blurring crowds into ghosts, stretching clouds into streaks. Variable ND filters, which rotate through a range, are convenient but can produce cross-shaped darkening at extreme settings. Cheap ND glass frequently is not truly neutral and casts magenta or green, which then has to be corrected in grade.