ND = Neutral Density
here are 3 similar shots taken by using my Pro 90 IS at Botanic Gardens.
Waterfall shot 1 (no filter used)
Shutter speed: 1/320 sec
Waterfall shot 2 (no filter used)
Shutter speed: 1/40 sec
Waterfall shot 3 (ND 8 and Polariser filters used)
Shutter speed: 0.8 sec
As you can see , by adding a ND and polariser , I can set longer exposure to shutter speed to give a smooth effect on the waterfall ...