It’s a device placed between the teeth that prevents the jaw from retracting when sleeping, causing the upper airway to be more open so the air can pass without difficulty.
If you have been diagnosed with obstructive sleep apnea — a condition in which relaxation of the muscles around the tongue and throat causes the tissues to block airflow to the lungs while you sleep — there are a number of treatment options to discuss with your doctor. Two of the most widely used and most effective are continuous positive airway pressure (CPAP) and dental appliances, or mouth guards. – WebMD