The labrum is  the articular surface (the area where the ball of the arm bone or humerus connects to the actual joint surface or glenoid cavity *the socket of the shoulder).   Its like looking at the Universal Joint on a car crankshaft.  The surface on the inside of the socket is where the labrum forms a secure ‘cuff’ for the bony humerus to insert into.  Like a ring of fibre that makes the articulation more secure, it helps to create a deeper rim.  Like the rubber ring on a preserving jar.