Anyone know what is a pleural effusion?

Fluid pleural space. The pleusa is a specialized tissue that lines the lung(visceral pleura) and the thorax(parietal pleura), every day it is estimated up to 9 liters of fluid could be exchanged in between the pleuras. This fluid traverses the pleural spaces and gets absorbed in the visceral pleuras. The process is seamless but conditions that affect the pleura or the lungs may result in fluid buildup in the space.
Fluid. Pleural effusion is a build up of fluid around the lung, and can compress the lung & cause breathing difficulty. The fluid can occur from heart failure, infection, tumor, etc.