ND Filters
There are several kinds of ND filters, but there are mainly two types; variable and non variable. Variable filters let you adjust the amount of stops the light entering the aperture is cut down by. On the other hand, non-variable filters reduce the light by a fixed number of stops. Cutting down the light combined with slower shutter speeds allows you to see motion in subjects which makes for more interesting shots.
When it comes to video, it helps avoid losing details to clipped highlights while working with wider apertures. They also come in a variety of sizes, which are indicated by the number on the side of the filter, for e.g. Ø 49mm
Now that you know what these filters are all about, try taking one on your next shoot!