These filters are a great way of doing slightly more creative things in daylight conditions, where normally you struggle to get long exposures.
ND stands for neutral density, so the filter is a neutral grey shade which greatly reduces the light coming into the lens but doesn't add any colour of its own.
This a 6 stop version by B&W, who make beautifully crafted filters, with a very robust brass frame. Therefore if you put it on your lens it will increase the time of the exposure by 6 stops.
Why would you want to do this?
Well, how do photographers get those lovely daytime shots of flowing waterfalls or breaking waves with that surreal misty effect? How do they make clouds race across the sky? They stop the lens down to a small aperture, choose a low ISO sensitivity and add a ND filter.
In dull conditions its possible to get exposures of 20 or 30 seconds at ISO 100 equivalent and stopped down to F16 or F22.
In bright conditions this 6 stop version isn't really enough...you need to go seriously dark and get the 10 stop version like my brother (who recently took some great long exposures in London) as below:
http://www.flickr.com/photos/mbphotography/3843684322/in/photostream/
The 6 stop is actually easy to use as the camera will still meter and there is enough light to compose in the viewfinder...autofocus still works too, though manual is much better for optimizing your landscapes anyway. The 10 stop is a different animal and you lose this functionality because the filter is simply too dark.
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