What does it mean when you are driving and the rear wheel is not turning?

If only one wheel is not turning, this would indicate a serious problem with that wheel. Depending on if it was a driven wheel or not, it could be a problem with the brake, the bearing, the differential or driveshaft.

Continuing to drive will likely cause damage to tyre, wheel, suspension or drive. It is also likely to have severe effects on the vehicles handling.