ND filters or neutral density filters reduce light intensity without changing color.
There are different brands with different filter levels, from 2 to 1024. Different values are doubled at each step. For DJI drones, DJI sells ND4 ND8 and ND16 for Phantoms, Inspire and Osmo, and Autel brands include ND4, ND8, ND16, ND 32 for EVO Lite series and EVO II series.
An ND filter acts like sunglasses would to your eyes, it blocks light coming into the lens like sunglasses blocks light coming into your eyes. ND stands for Neutral Density which means it blocks the light in a neutral way without changing the colour of the light.
You can compare this with photographing in a dark room compared to a well lit one; the first requires a much longer shutter speed (or higher ISO) in order to collect as much detail as you’d get in a well-lit room.
Why Do You Need One?
An ND Filter is perfect for use in landscape photography, especially when you want to achieve long-exposure effects such as a milky effect in the water, or to show the movement of clouds in the sky.
Water, and particularly waterfalls, are perfect examples of when you would want to use an ND Filter. Moving water creates a really nice effect when you use low shutter speeds (1/15th of a second and longer).