A posture collar is a rigid collar made of leather, metal or plastic. It is a form of bondage or restraint which limits the movement of the wearer's neck and head. At a minimum, it compels the wearer to keep their head up (hence, posture collar) and some collars can also limit side to side movement of the head as well.
As we can see from the illustrations on this page, a posture collar is generally wider in front than in the back. It effectively wedges itself between the wearer's chest and chin thus keeping the head high.
It is similar in some respects to an orthopaedic collar. In medical use, an orthopaedic collar is used to limit head movement to allow neck injuries to heal.
Posture collars can cause chaffing and, at least initially, a submissive wearing a lockable posture collar should be checked at intervals to see if this is occurring. Some collars have soft edges or trimming to reduce chaffing but it can still be a problem, particularly if the collar becomes wet through sweating during a scene.
Posture collars can also cause or trigger muscle stiffness or pain in the neck.