Polarizing filter: light so that most of it in one direction.
The polarized light passes through a matrix of liquid crystal cells. In a color display, there are 3 cells positioned properly for each pixel.
When an electrical current is applied to one of these cells, the molecules in the cell spiral.
The light coming out of the crystal is now polarized in different directions, depending on the strength of the current that was applied to the crystal.
The light is now passed through a red, blue or green color filter.
The final polarizing filter blocks all light that is polarized perpendicular to its preferred direction, the second filter will only pass through the light that is polarized in the correct direction.
Therefore, the brightness of the light is proportional to the amount of polarization twist that was applied by the liquid crystal's spiral.