Heart rate monitors, or HRMs, have the function of measuring, displaying, or recording an individual's heart rate. Consumer HRMs are often built into other devices (especially wearables) as wireless sensors.
Although widespread use of consumer heart rate monitors is during physical exercise, HRM sensors may also be active throughout the day, transforming heart signals into data for health and exercise reports and, in some cases, sending out alerts for elevated heart rate.
Current heart rate monitors may be electrical or optical. Electrical HRMs are made of two components: a monitor and a receiver. The heart's signals are read by the monitor's electrodes, which then transmit heart rate data to the receiver via wireless signal (Bluetooth, ANT+, or another low-power wireless communication technology). As for optical technology, the heart rate is measured with the help of LED lights emitted onto the skin, allowing for blood flow analysis. Considered to be less accurate than medical-grade HRMs, which typically employ the electrical current reading, optical technology is the most commonly used in multifunctional wearables (like smartwatches and fitness trackers).
Smartwatches are the most complete of wearables in terms of functionality, and many of them have an inbuilt heart rate monitor which can help you reach your fitness goals and keep your health in check (although, of course, wearables are no substitutes to a visit to the doctor's office). Fitness trackers do much more than track your steps, and several models include HRMs.