This is a swelling of a fluid-filled sac called the "subacromial bursa." It's in the shoulder, between a bony protrusion called the "acromion" and the rotator cuff. You have similar sacs near other large joints throughout your body. They act as cushions between your bones and your soft tissue. Normally they have a small amount of fluid inside them. But sometimes they can swell. We call that "bursitis."