The lungs are covered by a membrane, or lining, called the pleura, which has an inner and outer layer. The inner layer covers the lungs. The outer layer lines the rib cage and diaphragm, which is a sheet of muscle that separates the chest from the abdomen.
The pleura produces a fluid that acts as a lubricant that helps you to breathe easily, allowing the lungs to move in and out smoothly. Sometimes too much of this fluid can build up between the two layers of the pleura; this is called a pleural effusion.
Pleural effusions can be a symptom of cancer.