Light from the sun or even from a lightbulb comes toward you in vibrating waves. When it hits a reflective surface it shoots back at you which is why it seems so bright when you look at the lake on a sunny day. A polarization filter basically fixes this. The majority of reflected light waves come at a horizontal angle. A polarized filter is vertical only letting in vertical light waves. The majority of horizontal light waves can't make it through therefore eliminating them.
I just got a new pair of sunglasses for fishing. They said they were polarized, but I wanted to make sure. To do this all you need is your glasses and a computer screen. Turn the computer on and put the glasses up to it. Now turn the glasses 90 degrees. If they are polarized you won't see anything anymore. I love my new sunglasses.