Let's understand "the middle lobe of the lung is filtrated" piece by piece.
First, the following picture refers to the middle lobe of our lungs. (It also clarifies the other parts of the lungs, too.)
The word "filtrated" means to block off unwanted solid objects in air or water (really general definition). In the case of lungs, the middle lobe being filtrated justifies that there are no mucus accumulations stuck within the alveoli, thus making breathing easier.
Therefore, the phrase "the middle lobe of the lung is filtrated" means that the middle lobe is clear of any solid particles that may interrupt or decelerate the breathing process - meaning it's a healthy lung.