What does mild dependent atelectacis of lungs mean?

Collapse of air sacs: Atelectasis is a a complete or partial collapse of an are of lung. It happens when the tiny air sacs (alveoli) within the lung become deflated. 'mild' means that the changes are slight/minimal. 'dependent' means that the less inflated areas are in regions of the lung where gravity is pulling the tissue down - often related to the patient being in one position for a long time.