Glare - Glare is apparent when one object in your field of vision is much brighter than the rest and that brightness interferes with your ability to see other objects in your field of vision. It is concentrated light reflecting off a horizontal shiny surface, such as a car windscreen, sand, water, snow or asphalt roads. It reduces visibility and can make it uncomfortable, painful and even dangerous to carry on driving, cycling, skiing or just sunbathing.
Polarised Lenses - Polarised lenses eliminate glare, they do this by making use of the fact that lightwaves travel in random orientations and glare results from lightwaves travelling horizontally. By blocking horizontal waves and letting vertical ones through the lens virtually eliminates the glare, it lets less light through as a result, and so the tint on a polarised lens does not need to be as heavy as a non-polarised one to get the same light transmission.
Methods of Polarisation - the easiest method of creating lens polarisation is with an external film coating, however more expensive sunglasses sandwich a polarising filter between the layers of the lens which is a more durable method.