Heart rate is the number of heartbeats per minute. It can be detected manually, by palpating the pulse at the wrist or carotid artery.
A heart rate monitor is beneficial for monitoring heart rate during exercise. This shows the current heart rate, as well as the specified lower and upper limit. This allows for controlled heart rate training.
In addition to the classic heart rate belt, there are now also many fitness watches from Garmin and co. that offer an integrated heart rate measurement on the wrist. This is not quite as accurate, but is great for beginners. This allows you to use the functions effectively for several sports (road bike, mountain bike, running, hiking….) and in everyday life.