A normal shoulder consists of three joints. These are the glenohumeral joint (which is the primary joint that moves our shoulder and is in the form of a ball and socket), the acromioclavicular joint, the sternoclavicular joint, and the scapulothoracic joints between the chest cavity and the shoulder blade.
The rotator cuff consists of four muscles that connect the shoulder blade to the arm bone. These muscles are called rotator cuffs since they surround this ball-and-socket joint in the form of a cuff. The rotator cuff muscles press the ball into the socket during shoulder movements and provide a stable platform for circular movements during this activity. Of these cuff muscles, the most commonly torn one is the supraspinatus muscle or tendon.
The shoulder joint is the most mobile joint of our body in all directions. This motion capability also paves the way for shoulder dislocations.
The head of the shoulder is like a golf ballon a tee as shown below
In other words, we can think of the shoulder joint as a balloon kept in balance by a seal, which has a gap, and where the ball (larger) and the socket (smaller) are not compatible in size.