heart rate monitors work by measuring electrical signals from your heart. They are transmitted to a wristwatch or data center. Many models let you analyze data via a computer and having that data allows you to interpret your workout and better understand the benefits from your exercise.
You can get a rough estimate by monitoring your own pulse the old-fashioned way – feeling it in your wrist or your neck – but that can be disruptive to your actual workout. An electronic heart rate monitor can give you accurate, real-time data.