Do you even know what that is? Do you know what a regular ND filter is? Let's see if we can explain.
ND stands for Neutral Density. And it basically acts like a light restricting filter for your lens. By limiting the amount of light that comes into the lens, you can intentionally slow the shutter down in brighter light conditions that you normally would not be able to. So you can take flowing pictures of water falls in brighter conditions.
A variable ND filter, is almost like you stacked several ND filters in place. It is fully adjustable. You can remove anywhere from 2 to 8 stops of light just by rotating the filter. The more you turn, the more light is removed.
With this technique, you can make a busy city street or highway, turn into an empty urban scene like what you see with the photos from photographer Matt Logue below. He made the street of LA completely devoid of human life.
So what does one of these things look like? Like this...
Here are links to both.
Tiffen 77VND 77MM VARIABLE ND FILTER for Camera lenses
Fotodiox 7 Metal Step Up Ring Set, Anodized Black Metal 49mm to 77mm