Light travels following a wavelike path. Water waves on a pond travel in a similar manor however, water waves undulate up/down whereas light waves freely vibrate in every possible plane. Such light wave behavior is said to be un-polarized. When light waves are reflected from some surfaces, they often become polarized. Polarized light wave vibrate however the direction of their undulation is altered.
Light reflected from non-conductive surfaces such as glass undulate mainly in one plane. When first studied, scientist falsely concluded that light waves must have a north and south pole, like a magnet. While false, the term polarized and un-polarized stuck. When light rays are reflected from the polished surface of glass windows and showcases, it will likely be polarized. Polarizing filters pass light waves that vibrate in a single plane and block light waves that vibrate in opposing planes. Consider a jump rope passed through a fence with a missing picket. The jump rope when plucked only vibrate up/down and not side-to-side.
This being the case, when photographing objects behind glass, we can mitigate impairing reflections by mounting a polarizing filter and rotating it so that annoying reflections are rejected.