A polar-ring galaxy is a galaxy with a circle around it of stars and gas. Those that clearly show the pattern have a ring on a plane at odds with the galaxy, polar indicating the case where the planes of the ring and galaxy are at right angles.
The ring is thought to be due to a previous interaction with another galaxy. One possibility is a galaxy merging with the center of another, leaving a surrounding ring. Another possibility is that the ring is the remnant of a tidal arm, stripped from a passing galaxy.