It’s the sense that tells you where your body is in space and how it is aligned. It’s more highly tuned in some people than in others.
I remember a conversation with Alex years ago. He was developing some front rotating skills on his floor routine, front flips. I observed that they must be harder to stick because with backwards rotation you can look ahead and see when to push out your feet to make the landing. He answered that it didn’t make any difference, his eyes were closed the whole time anyway. He was doing all those twists and flips and suddenly he’d land. With his eyes closed. “I just know”, he said. That would be proprioception, but to a degree few of us have ever known.
Even though we don’t have that sense in that way, we all have it to some degree. It’s what keeps us upright when we move about. We know where we are and what we need to do to stay the way we want to be. That sense degrades however as we get older. The challenge, or one of the challenges as we get older, is to maintain as much of that spatial awareness as we can, as long as we can.
So, what’s the point? Why am I bringing this all up now? Proprioception? No reason. Why do you ask?