Proprioception is commonly referred to as joint position sense. Simply put, it allows your body to know where your joints and muscles are positioned and how they are moving without you looking at them. There is actually more to it than that! It is key to our ability to move as it is a subconscious sensory-motor control system that co-ordinates all our movements and posture. It also protects our muscles, joints and bones from injury. The nervous system receives information from sensors in muscles, tendons and ligaments and integrates this with information from the vestibular, visual and tactile sensory systems to monitor the body's internal state, position and movement.

It is proprioception that will make you pull your hand away from something that's too hot or stop you twisting your ankle when slipping off a step.
Controlling muscle tone is one of it's most important functions but is often overlooked.

Proprioception is based on reflexes:

flexor reflex
This is the flexor withdrawal reflex. It makes you lift your foot up when you stand on a nail by contracting your flexor muscles and inhibiting the extensor muscles, before you've even thought about it.

 myotatic relfex
The myotatic reflex, also known as the knee-jerk reflex, is caused by rapid stretching of the muscle spindle cells. These then cause the muscle to contract and at the same time inhibits the antagonist (opposing muscle).

The flexor withdrawal reflex can override the myotatic reflex and if the irritation is not or cannot be removed it may become the permanent state. This leaves certain muscles inhibited.