The shoulder joint socket is extremely shallow and very unstable. This means that shoulder bones are not held in place adequately. Thus, the shoulder joint has a cuff of cartilage, called a labrum, that forms a cup for the end of the arm bone to move within.
The labrum circles the shallow shoulder socket to make the socket deeper. This cuff of cartilage makes the shoulder joint more stable and allows for a wide range of movements.
The labrum tissue is susceptible to injury with trauma to the shoulder joint. If you've injured your shoulder, a labral tear may be the culprit.