When you inhale (breathe in), air enters your lungs, and oxygen from that air moves to your blood. At the same time, carbon dioxide, a waste gas, moves from your blood to the lungs and is exhaled (breathed out).
Every time you breathe in air, your diaphragm tightens, moving downward to make space in your chest. Your lungs expand, pulling air in through your nose and/or mouth. That air then moves down your trachea, through your bronchi and into the bronchioles, where it enters your alveoil.