When you breathe, air travels through passages in your nose and throat. When these air passages are wide enough to let air flow freely, you breathe normally.
The septum is the wall that divides the left half of the nose from the right half. Turbinates are ridges in the nasal passage.
Air flows past soft, flexible structures where the mouth meets the throat: the soft palate, uvula, tonsils, and back of the tongue. Throat muscles hold these structures in place. While you sleep, the throat muscles relax a bit. But they normally stay tight enough to keep the airway open.
Front view of nose and mouth.
Side view of nose and mouth. During normal sleep, air flows freely past the structures in the throat.