A baulk ring is a gearbox / transmission component commonly found in most modern car gearboxes.
It is a mechanism which helps ensure that gear changes are smooth and quiet by preventing the gears from being engaged until both components coming into contact are travelling at the same speed.
The synchro cone is the component which actually speeds up or slows down the gears before they are engaged while the baulk ring stops the engagement taking place until the speeds match.