Individual photons whose electric field vectors are all aligned in the same direction.
Ordinary light is unpolarized because photons are emitted randomly –
Laser light is polarized because photons are emitted coherently.
When light passes through a polarizing filter, the electric field interacts more strongly with molecules having certain orientations.
Polarization is called state selection: photons have only two states …, so light passing through the filter separates into only two beams.