The shoulder is a complex multi-directional ball-and-socket joint which allows movement in many directions. Shoulder instability and dislocation occur when the shoulder capsule is stretched or torn, and/or when the labrum is detached from the glenoid. The more the labrum is stretched or torn, the more unstable the shoulder becomes.
The shoulder may pop out of joint so frequently that it tears the inferior glenohumeral ligament, an injury called a Bankart lesion, named after English orthopedist Arthur Bankart.
In most cases the joint may then be repaired arthroscopically, as shown in the following animation.