What polarization describes
Light is a transverse electromagnetic wave, and polarization refers to the orientation of its electric field oscillation. Unpolarized light from sources like the Sun contains all orientations mixed together. A polarizing filter transmits the field component aligned with its axis and absorbs the perpendicular component, producing linearly polarized light.
Light can also be circularly or elliptically polarized when two perpendicular components are out of phase, but linear polarization is the case a single ideal filter produces.
Malus's law
When already-polarized light passes through a second polarizer (an analyzer) set at angle theta to the first, the transmitted intensity follows Malus's law: I equals I-zero times cosine squared of theta. At zero degrees all light passes; at ninety degrees the filters are crossed and transmission falls to zero. An unpolarized beam hitting an ideal polarizer loses half its intensity regardless of orientation.