ETOPS means Extended-range Twin-engine Operational Performance Standards. It is a certification that permits twin engine aircraft to fly routes which may, at the time, be 60 minutes flying time from the nearest airport that is suitable for an emergency landing.
More informally known as Engines Turn or Passengers Swim.

Using your example, MIA-CUN is a one hour 45 minute flight, which is definitely within the range for either a return to MIA if less than half way, or continue to CUN if more.