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ND filter reduces the light intensity reaching to the sensor or film through the lens, which effectively slows down the shutter speed. This is useful if you want to capture water motion or add motion blur to your image.
When using ND filter under bright condition, it gives you a greater flexibility to change the aperture and shutter speed. It allows you to use
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a larger aperture for the desire DoF,
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a slower shutter speed to maintain flash sync or to create motion blur.
The down side using ND filters is that they do not reduce the intensity of all wavelengths equally. This can sometimes create color casts in the images, particularly with inexpensive filters. Most popular filter brands such as B&W, Cokin, Lee and Hoya offer a variety of ND filters. Some common ND filters being the ND2, ND4 and ND8 which offers reduction of the light intensity by 1-3 stops.
So…. what can you do with ND filter?
Here is an image captured using an ND filter to slow down the shutter speed to 30s.
What does it look like without an ND filter?
A much more calming effect using an ND filter to capture the water motion. 
and of cause you will need a solid tripod to capture the water motion. Behind the scene of my travel tripod (which only weighs 1.5kg) holding up the Nikon D700 and 24-70mm f/2.8.
