Your shoulder is made up of several joints combined with tendons and muscles that allow a great range of motion in your arm. One of these groups of tendons, the rotator cuff, is a frequent source of pain in the shoulder. When you raise your arm to shoulder height, the space between the acromion and rotator cuff narrows. The acromion can rub against (or “impinge” on) the tendon and the bursa, causing irritation and pain.
More on Shoulder Impingement from the American Academy of Orthopedic Surgeons (AAOS).