Neutral Density Filters (commonly referred to as ND Filters) are filters placed in front of the lens to reduce the amount of light entering the camera. This variable ND filter will replace all your traditional ND filters. You can the change the density from ND2 to ND400 as you turn the outer filter ring.  It can also be used to create fade-in and fade-out effects while shooting videos because it is totally dark when it is turned at ND400 range.