The olecranon is the pointy bone at the tip of the elbow. The bursa is the thin sac of fluid that lies between this boney tip and the skin. It helps the skin slide over the bone smoothly. Normally, this sac has only a tiny bit of fluid inside of it and lays flat. However, the bursa can become irritated or inflamed and fill with extra fluid. When this happens, a painful swelling develops at the back of the elbow. This swelling is olecranon bursitis.