ND filters are tremendously useful for landscape photographers, giving you far more creative control over your images and the abiilty to make photos that simply would not be possible otherwise. If you are new to landscape photography, check out this neat video that features an experienced photographer showing you some of what you can accomplish with one.
Coming to you from Gary Gough, this great video will show you some of what you can accomplish with an ND filter. Essentially, ND filters are simply dark glass that reduces the amount of light passing through the lens and reaching the sensor or film, allowing you to use longer shutter speeds. They come in different strengths that cause different reductions in the amount of light. The most common options are three-, six-, and 10-stop filters, though as you can see, you can also use options like 15 stops, as Gough did for this shoot. This allows you to do things like smooth out turbulent water or create fascinating cloud trails across the sky. Check out the video above for the full rundown from Gough.
And if you really want to dive into landscape photography, check out "Photographing The World 1: Landscape Photography and Post-Processing with Elia Locardi."