The labrum is a rubbery cartilage ring much like a washer or gasket that surrounds the socket (glenoid) of the shoulder. The labrum is a critical structure because it acts as a stabilizer of the shoulder as well as an attachment point for many of the ligaments around the shoulder. The biceps tendon also attaches to the superior labrum. If the labrum is torn or detached from the glenoid, the ligaments or tendons that attach to it are thereby detached making the shoulder unstable, painful, or both.
In addition, if a piece of the labrum is torn loose, it will often get caught between the ball and socket and cause a sensation of grinding, popping, and pain. This is typically most noticeable with rotational movements of the arm such as throwing, reaching overhead, etc.