Proprioception as it relates to exercise is the ability of the brain to recruit the proper muscle groups needed to counteract any outside force. Any robot designer will tell you what an amazingly complex task it is to simply walk. The brain is able to propriocept, or assess, how the body is positioned and properly process that information in order to control the body into the next position. For proprioception the brain gets its stimuli from the muscles and also from the eyes and ears.
Proprioception is often synonymous with balance.
Challenges to proprioception or balance happen in every day life such as slipping over a banana peel or tripping over a misplaced object. Any time you get thrown off balance is an instance where the brain is especially challenged to propriocept. The stimulus suddenly changes and your brain needs to counteract and compensate. Hopefully, your brain processes these new stimuli quickly enough, to recruit muscle groups which hopefully will respond. Hopefully is the key word here.