During sleep, the muscles in the throat loosen (which narrows the airway). As the person breathes in and out, it causes the soft tissues to vibrate and make noise. Snoring can be a symptom of sleep apnea but not everyone who does it has sleeping disorder. Sleep apnea is when your breathing stops or slows down for a short time and then starts again.
Factors that can contribute to it, include age, weight, alcohol consumption, sleep position, and allergies.