What exactly does it mean to have a small pleural effusion?

Pleural effusion : Small amount of fluid collection around the lung in chest cavity.

Some fluid: The thoracic cavity contains the lungs heart esophagus. There is a space between inner chest wall and lungs which normally contains a small amount of body fluid. In some conditions, more fluid accumulates, and that is called pleural effusion. If it is bloody that can be a hemothorax. If pus, that is an empyema. Etc.