A polarizing filter is one of those nifty devices that help block haze, reduce light glare and increase contrast in clouds. Its difficult to describe. Some say its like having a set of vertical blinds that you can close part way, blocking out the glare on a scene. A polarizer works because of the way light waves travel.
Polarizers are great for shooting water or glass because they block those annoying reflections that plague photographers. There are two kinds of polarizing filters, linear and circular. Circular polarizers are generally compatible with all sensor types on all cameras. This is why they are more recommended than the linear type.
Polarizers are just one of those "must have" filters that are worth an investment.
If you want to read more about the science behind polarized light, check here. Happy shooting!