Rear steer is the rear axle sitting or moving out of perpendicular to the chassis centerline, so the back of the car effectively steers itself and goes tight or loose with no input from the driver.
Hyper Racing puts it plainly: “Rear steer happens when the rear axle is not perpendicular to the center line of the chassis.” Direction matters — with the right side forward “the chassis will feel as though it has understeer or tight,” and skewed the other way it “will feel like it has more oversteer or loose.” Two separate conversations hide under one name. Static rear steer you build in “by simply lengthening or shortening the rear locating mechanism.” Dynamic rear steer shows up as the suspension works: over bumps, from spring deflection under cornering and acceleration g-forces, and from chassis roll. On a Z-link, Hyper Racing says “more rear torsion arm angle means more rear steer” and “a longer top radius rod means more rear steer.”
Rear steer is why a car can go tight after a change at the back that had nothing to do with the front, and it will send you chasing the front end for no reason.