Between the two pleura’s, there is a fluid (= pleural fluid) to lubricate the two surfaces. This lubrication is necessary because when one breathes, as the lungs expands or deflate, the two pleura will rub against each other.
Try this at home!
Take two hard but flat surfaces. Two glass plates are ideal but you can also try this with two flat plastic surfaces. Place them on top of each other and rub one on top of the other. Notice how rough this actually is. Now place some water between the two plates and rub again: Aaah! This is much smoother!
If the pleural space is dry, as in pleuritis (= infection of the pleura), the two pleura rub against each other with each inspiration without the lubrication of the pleural fluid and this is painful!