The ND filter or gray filter as it is also called is screwed onto the lens to block the incident light. The function is similar to a pair of sunglasses. If you look directly into the sun with your unprotected eyes, it does not take long until all you can see is white. The sensor of the camera is similar to the iris of the human eye and can also be blinded if hit by light that is too strong. As a result, the image becomes too bright or unusable.
ND filters are often used in landscape photography when longer exposure times are necessary. For example, when taking a picture of a landscape where you want to achieve a smoothing effect, or even with running water such as a waterfall. However, the ND filter is only necessary if you can not expose long enough despite setting the desired maximum f-number, without the image being too bright or even burned out.
Even with portrait photography with a tendency to an open aperture and short exposure time, the ND filter can make sense. Especially with outdoor shootings in the morning or evening in low sun, the light on the sensor can be extreme. Taking pictures against the sun is possible with the ND filter. Great moods can be captured that way. It always pays to have a gray filter with you when taking photos outside in good weather.
ND filters are available in different strenghts and sizes. However, there are also variable ND filters, where you can adjust the intensity at any time, instead of screwing several ND filters on top of each other. The advantage of an adjustable or Vario filter is that you only need one filter.
A recent video shot with Schneider Vari ND filter Kit. The video shows how the light can be controlled through the rotation of the filter (from 0 -10 stops) without changing F-stops. And I was able to change the amount of ND I needed while I was shooting, since the sun was going in and out of the clouds, I was able to adjust the amount of ND needed without having to change f-stop or shutter speed or change out to a different filter.