Ever see a photo with a beautiful blue sky?  They probably used a Circular polarizer like this one to get those results.

Light rays which are reflected become polarized. Polarizing filters are used to select which light rays enter your camera lens. They can remove unwanted reflections from non-metallic surfaces such as water or glass and also saturate colors providing better contrast. The effect can be seen through the viewfinder and changed by rotating the filter. The filter factor varies according to how the filter is rotated and its orientation to the sun.

Multicoated to minimize reflection at the filter surfaces which reduces flare and ghosting.