Your airway is like a hose and the water that runs through it is like the air that you need to live.
Things such as swollen tonsils and adenoids, nasal polyps, swollen turbinates, or even your tongue can restrict the flow of air.
When the airflow is restricted your lung get less oxygen. This is called a hyponea. Where the oxygen content of the blood (Called Oxygen saturation) falls below 95%.
When the airflow is completely cut of you go through and Apnea. Literally you stop breathing just for a few seconds.
There have been documented cases of people that have stop breathing and due to their cardiac demand have had heart attacks during the night.