The lungs are covered by a lining (membrane) called the pleura. The pleura has an inner and outer layer. The inner layer covers the lungs. The outer layer lines the rib cage and diaphragm, which is the sheet of muscle that separates the chest from the tummy (abdomen).
The pleura makes a fluid that acts as a lubricant. This helps you to breathe easily, by 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.