Light waves from the sun bounce and reflect or refract light in every direction. Polarized light is reflected light that is organized in one plane of direction. An example in nature would be standing on the beach looking at a relatively calm ocean. The glare of the sun high above reflects off the water in every direction that makes it hard to look at the water without squinting or sunglasses or more likely both. That glare off the ocean's surface is light that cannot penetrate the ocean's surface and is reflected back. Water does this well, as do snow, roads and other shiny, bright, reflective surfaces. The glare of light you experience in these situations is horizontally polarized.
Uniqueness of Polarized Lenses
Polarized sunglasses neutralize that horizontal reflection of light by screening it through vertically oriented polarizing filters embedded in or applied onto the sunglasses lens. Only the part of the light wave that is aligned with the microscopic openings in the filter can pass through. Everything else is absorbed by the film on the lens. So the most intense light, that matches the original horizontally orientated light wave, will be blocked, and the glare will be muted significantly.
Benefits to the Wearer
This blocking of the harshest reflected light allows a crisper field of view without the painful brightness that makes you squint and your eyes water. This is critical for professions and sports that are conducted in and on reflective surfaces, especially professional sailing, boating, fishing, snow sports and car racing.