On polarized sunglasses, the filter creates vertical openings for light. This means that only light rays that approach your eyes vertically can fit through those openings. The lenses will block out all the horizontal light waves bouncing off a smooth pond or a shiny car hood, for instance.
As a result, the image you see with polarized lenses is a bit darker than usual, but objects look crisper and clearer, and details are easier to see. People who use polarized sunglasses for long stretches of time often say they are less tired than usual after hours of battling sun glare. People who fish, in particular, find polarized sunglasses drastically cut the glare and help them see into the water.