Neutral density filter reduces the amount of light going into the lens, so you can take long exposures even when the light is bright.
Long exposures blur anything moving, like water, clouds, or people. This can be very useful for making choppy water look smooth, making clouds streak, or getting rid of people at a tourist attraction.
These types of filters usually cost up to $200 (especially on big diameter lenses), but with this simple hack,... more