A pleural effusion is a life-threatening condition where fluid, generally from the lymphatic system, but occasionally blood, drains into the empty cavity between the lungs and the ribcage. As a result, the lung cannot expand into the cavity and the patient becomes unable to breathe.
Treatment is to withdraw the fluid with a large needle and a syringe, but if the condition continues, this must be done frequently. As such, it's important to establish the underlying condition that is causing the fluid to fill the cavity.