When light is incident on an optical filter at a non-normal angle of incidence, the polarization of the light can be described by two orthogonal vector components associated with the orientation of the electric field of the light wave.  The polarization is referenced to the “plane of incidence,” or the plane that is parallel to the normal to the surface of the filter and contains both the incident and the reflected light rays.  The polarization component that is perpendicular to the plane of incidence is called the “s” component, and the component that is parallel to the plane of incidence is called the “p” component.