Olecranon bursitis or elbow bursitis is inflammation of the bursa (fluid-filled sac). This bursa is present behind the elbow joint (olecranon process that is a pointy bone). You can feel a squishy lump on the backside of your elbow.
Generally, it occurs due to direct below on your elbow or you are exerting too much pressure on your elbow against a hard surface.