A Polarizing Filter (aka polarizer) is a filter which fits in front of your lens and blocks unpolarized light. Without getting into a physics discussion, you can say that a polarizing filter will cut unwanted reflections in your scene. This filter can cut reflections and glare from windows, glass, leaves and other non-metallic surfaces. Most importantly, it can cut reflections from water droplets in the sky to give you much richer blue.
Polarizing filters are considered a must for many landscape photographers.