The labrum surrounds the acetabulum (socket) of the hip joint. It helps deepen the socket that the femoral head (ball) sits in, providing a suction cup like affect of the ball in the socket. 

A tear of the labrum can occur due to a dislocation or subluxation injury, repetitive motion that causes the labrum to wear, boney abnormalities such as femoroacetabular impingement or dysplasia, or from osteoarthritis.  The labrum can also detach, tearing away from the acetabulum.

A labral tear or a detached labrum causes pain with flexing the hip, while rotating the knee toward the opposite leg and crossing the knee over the middle of the body. This motion causes a "pinching" of the torn or detached labrum, re-creating a person's pain.