Think back to the wave model of light. Light is a
transverse wave, which means it vibrates perpendicular to the direction it
travels. Even though light has two waves, electric and magnetic, for this
discussion of polarization, we'll focus on just one of the waves. Now our
light waves look like this:
Imagine rotating the wave around its axis. As it moves, we observe different polarizations of light. The polarization of light is the direction the wave is vibrating in. Usually, light is composed of light of all polarizations. It is possible to make a filter that only allows light of one polarization through. Light after passing through one of these filters is said to be polarized.
Looking through a polarizing filter makes everything look darker because it eliminates most of the light. That is why polarizing filters are often used in sunglasses. They are also used in certain types of 3-D displayes so that one output screen can be split between two different eyes. The user wears glasses with a horizontally polarized lens over one eye and a vertically polarized lens over the other. The screen outputs the image for one eye in vertically polarized light (so the other eye won't be able to see it) and the image for the other eye in horizontally polarized light. The lenses only allow one of the images through for each eye, and the user then thinks he/she is seeing a 3-D image coming out of the screen.