Light is a vibration of an electric and a magnetic field. If you could watch light waves coming toward you, you would see that they actually vibrate in many directions. We say the light is polarized when it is forced to vibrate in only one plane.
Polarizing sunglasses block out all but one of the vibrations of light so that considerably less light reaches your eyes after passing through the lenses. This is kind of like passing a rope through a picket fence and shaking the rope in many directions. The only wave that passes through the fence is vertical.
