When a lens is polarized, it has a built-in filter that blocks bright, reflected light. This intense light is known as glare. When glare is reduced, your eyes feel more comfortable and you can see your surroundings more clearly.
Sunlight scatters in all directions. But when it strikes flat surfaces, the reflected light tends to become polarized, meaning the reflected rays travel in a more uniform (usually horizontal) direction. This creates an annoying, sometimes dangerous intensity of light that can reduce visibility.