Fluid filled sac...
That can develop over any tendon or joint in the body.
Usually, joint is damaged in some way, such as with osteoarthritis
, where there is some destruction of the integrity of the joint space. Mostly, not painful or disabling, though may be unsightly. Exceptions would include one that is within the lower lumbar spine impinging on nerves; or knee 'baker's cyst' causing pain & swelling (r/o dvt).