Light rays which are reflected by any surface become polarized and polarizing filters are used to select which light rays enter your camera lens.
They allow you to remove unwanted reflections from non-metallic surfaces such as water, glass etc. They also saturate colors and increase contrast.
Polarizers come in two variations but have the same effect. Linear polarizers work with older camera meters; Circular polarizers are designed for today's newer split-beam metering systems used in auto focus cameras. Check your camera's manual
Extra thin filters are narrow construction with ultra thin glass. They are designed to avoid vignetting with today's larger wide angle lenses.