These two contraints - what you want to do with the aperture or shutter speed, and
what the camera needs to do to give you a good exposure - sometimes come into
conflict. If you take a picture of moving water, you often want a slow shutter speed
(1/30th to 1 second, depending on the effect), but if it's a sunny day, you'll overexpose
at that shutter speed.
Enter the neutral-density filter. With the the filter, you get a 3-stop (3 factors
of 2, or 1/8th) decrease in the amount of light that comes in. That means that
instead of an exposure of F16.0 at 1/125th of a second, you can either open the lens
up to F2.0, or lengthen the exposure to 1/15th of a second.
I used this last night on some fireworks, just so the CCD sensor doesn't get overloaded.
The cool part is that it's just a setting in the recording menu, and everything else
works the way you'd expect.