Proprioception is one of these terms that used a lot and many try to improve it.
The fastest way to explain it is that it is the collection of information that goes to your brain from our body. Our eyes, ears, joints, muscles, skin, ligaments send constant information through our nervous system to tell our brain about the position of our in space and time. This information is then used to execute the movement.
Information goes into the sensory cortex and goes out through the motor cortex to create movement.
The quality of the movement depends on the quality of the information in and out. In order to truly prevent injury and to rehabilitate one, we must focus on good communication between important parts of the body and the brain. What is important is what takes up the most space on the cortex.
The quality of information and movement can be altered by metabolic problems, pain, psychological stress, scars, poor habits..