A Neutral Density filter reduces the amount of light passing through the camera lens without changing the color of that light. It is especially useful in bright light conditions to help prevent overexposure. It can also help you get the shutter speeds down for more blur effect with waterfalls and any moving items in bright light.
As you can see in the pictures the filters are stackable, this will give less light. There is a 2 and a 4 filter in the picture the 2 on the left and the 4 on the right. In the center you can see the effect of the 2 together, which is a 6. ND-2 absorbs 1 f-stop, ND-4 absorbs 2 f-stops, and ND-8 absorbs 3 f-stops.

By Bill Huber