Specifically made for the DJI Phantom 4, this ND4 filter reduces the amount of light coming into the sensor. It gives you the freedom to create your own work by allowing more control of the camera's aperture, exposure time and sensor sensitivity settings. 
Put more simply, the ND4 filter helps avoid overexposed photos.
The ND4 filter allows 1/4 of incoming light through, equal to a 2-stop filter.