The arrow points at the infrapatellar bursa. When it is inflamed, it becomes more balloon-shaped and the overlying skin becomes reddened and shiny.
Kneeling and bursitis
A bursa is a lubricated pouch or sac in regions where there is a lot of movement between muscles and bone. If uncommon pressure is repeatedly put onto the area of a bursa - what we call 'overuse' then the lining of the bursa may become inflamed. Two of the most common types of knee bursitis are 'clergyman's knee' and 'housemaid's knee', both from excessive kneeling, but in different body positions.