This is an excessive build up on fluid between the pleura - the space that surrounds the lungs. The area of fluid is clearly visible on an X-ray and usually appears to be white. The fact that it is indeed fluid is shown by the meniscus (curved area) where it meets the sides of the lung. Fluids may include blood or pus. Seeing fluid such as this may indicate infection.
Symptoms such as coughing, shortness of breath, chest pain can often be observed.
Treatment includes removing the fluid, preventing the fluid from building up again and treating the underlying cause (e.g. leakage, lung injury, inflammation).