Sleep apnea happens when the soft tissues at the back of the throat collapse and block the upper airway as we sleep. When we're awake and upright the muscle tone and vertical position stops this happening but when we're asleep, we don't have that.
People who snore very loudly may stop breathing in their sleep and will start again when the brain detects low oxygen levels and wakes the sleeper - this happens in sleep apnea. It can happen many times each night (although the sleeper isn't aware of waking) and the broken sleep and chronic low oxygen levels have serious health implications.
Being overweight (and therefore having extra tissue at the back of the throat and in the neck) is one cause. As is the way some people's jaw structure is formed.