Is a pleural effusion serious? Is it usually a sign of cancer?

Sometimes. Pleural effusions can occur for a variety of reasons. Cancer is a cause of pleural effusion. How serious it is generally depends upon the underlying cause and the amount of fluid. Temporary relief can be achieved with drainage of the effusion (called thoracentesis).
Several causes. Pleural effusions can be caused by infections, congestive heart failure and sometimes cancer. Asbestos exposure and autoimmune diseases are other causes. Sampling the fluid and having it analyzed is recommended.
Not necessarily. Cancer of the pleura, mesothelioma is uncommon. Metastatic cancers to the lung can cause pleural effusion. It my also be due to heart failure, lung inflammation and lymphatic obstruction. Pleural effusion without an obvious cause is considered to be due to tb, unless proven otherwise.