Snoring (not to be confused with sleep apnea) is the sound that's created when the tissues in your throat relax enough to the point that they partially block the airway and vibrate every time you breathe in and out. And according to Rodriguez, things that make your airway (the trachea) narrower, like sleeping on your back, only make the airway narrower and increase your likelihood of snoring noisily.
Other factors that minimize your airway and make you more likely to snore are having tonsils, going to bed extremely tired, drinking alcohol, and taking muscle relaxers or sleeping pills. You can also be genetically predisposed to snoring, like by being born with a small airway, having softer neck tissue, or being prone to gaining weight in your neck area. After menopause, women also tend to lose collagen in their skin, decreasing the elasticity in their neck and making snoring worse, which is why some women develop sleep apnea after losing high numbers of estrogen.
Sometimes, the walls of the throat collapse completely so that the airway is completely blocked, and that's the difference between snoring and sleep apnea.