The shoulder is the most mobile joint in the human body. It is a ball and socket joint- the ball is made from upper end of the arm bone (humerus), and the socket is made from part of the shoulderblade (glenoid). However, the wide range of movement of the shoulder comes at a price - it is susceptible to dislocation.
There are many structures in the shoulder joint that provide support and help the ball stay in the socket. These include ligaments, tendons, muscles and the fibrous capsule around the shoulder joint itself.
Shoulder instability occurs when the supporting structures of the shoulder joint aren't doing their job properly. For various reasons, the ligaments, tendons, muscles or joint capsule become too loose around the shoulder, and it becomes prone to popping out of place (dislocating).
50% of all joint dislocations involve the shoulder, particularly in young people.