Neutral density filters reduce the amount of light getting to the cameras sensor. Why on earth would you want to do that? well theres several reasons but the main one is to have a longer shutter speed.
Longer shutter speeds are useful in photography to create effects like this.
Because you have effectively got sunglasses on the lens you can keep the shutter open for much longer while the cars pass over the bridge, creating these streaks of light
However the main reason we have them on our drones, is for filming purposes. Drone filming often involves movement and when we see movement in real life, we see motion blur.
The only way we can create and control motion blur in video is by controlling the shutter speed and thats where these filters come in