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."
The rotator cuff is a group of muscles and tendons in each shoulder. It holds your upper arm bone in your shoulder socket. It keeps your arm stable while allowing it to lift and rotate. Too much stress on the rotator cuff can cause a tear. This can be a painful injury.