Polarized Light is light that has been made to vibrate on a single plane.
The Physics Classroom: Polarization
In the figure below, Polarizer 1 forces the light to vibrate vertically (up and down) only, getting rid of the horizontal vibrations. The second filter, Polarizer 2, only allows horizontal vibrations through. Since there are no more horizontal vibrations, Polarizer 1 stopped them already, the result is no light getting through after the second filter.
Sunglasses: Polarize light to help protect your eyes.