The fluid you are speaking about is called synovial fluid. It is the fluid that fills the cavity created by the joint of the femur and the tibia, fibula, and the patella, commonly known as the kneecap. The purpose of this fluid is to act as a shock absorber for the joint and to help serve as a lubricant to keep the bones from grinding against each other. The cartilage that encapsulates the end of the long bones coming into the joint have a small area between them that is filled with this synovial fluid. Normally, this fluid keeps the bones from contacting each other fully, thus minimizing the impact the full weight of the body would have on the joint. Synovial fluid has a very slippery feel and texture to it, as does the cartilage and ligaments that bind the joint together. The whole purpose of all these materials is to provide a more liquid operation to the knee joint as a whole. When the cartilage or synovial fluid is worn away, the person suffers pain in the joint as a result of the bones wearing away at each other.
synovial fluid. it acts a lubricant to reduce the frinction between the two bones.