The SRB 0.9 Full Neutral Density Filter is a square resin filter that reduces the amount of light reaching the camera lens by 3 Stop, meaning the camera can perform with a higher aperture to produce a result far closer to how it appears to the eye.
A Neutral Density filter allows a photographer to control the exposure in an image very easily. The filter stops light reaching the camera sensor, therefore allowing the camera to shoot with a higher aperture for a longer amount of time.
Instead of changing the aperture to reduce the amount of light in the image, simply add an ND filter, and then adjust the exposure to the setting of your choice.