Light reflected from electrically non-conductive surfaces is more or less polarized depending on the angle of incidence, i.e. its oscillations are not evenly distributed in all directions, but are preferentially aligned in one plane. A polarizing filter allows such light to pass through to this plane, weakens it or blocks it, depending on the arrangement of the filter. A linear polarizing filter is recommended for your SLR camera if it does not use partially mirrored disks or prisms ("beam splitters") in front of the AF sensor and in front of the exposure metering line.
