Light rays which are reflected by any surface become polarized and polarising filters are used to select which light rays enter your camera lens. PL (Linear Polarizing) and PL-CIR (Circular Polarizing) filters have the same effect, but it is important that you choose the correct version for your camera.
They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass etc. They also enable colors to become more saturated and appear clearer, with better contrast.
This effect is often used to increase the contrast and saturation in blue skies and white clouds. HOYA's polarising filters do not affect the overall color balance of a shot.