A part of the scapula, called the glenoid, makes up the socket of the shoulder. The glenoid is very shallow and flat. The labrum is a rim of soft tissue that makes the socket more like a cup. The labrum turns the flat surface of the glenoid into a deeper socket that molds to fit the head of the humerus. The rotator cuff connects the humerus to the scapula.
Tendons attach muscles to bones.
Muscles move the bones by pulling on the tendons. The rotator cuff helps raise and rotate the arm. As the arm is raised, the rotator cuff also keeps the humerus tightly in the glenoid of the scapula . The soft labral tissue can be caught between the glenoid and the humerus. When this happens, the labral tissue may start to tear.