Your shoulder joint is shaped like a ball and cup. The ‘ball’ is the top of your arm bone. The ‘cup’ is on the outer edge of your shoulder blade (scapula). When you lift your arms above your head, the scapula rotates to allow more movement of your arm.
If your scapula is not rotating well, then you will end up with an ‘impingement’ of the shoulder joint. This essentially means that the arm bone will crash into the scapula.