Heart rate (HR), also known as pulse, is the number of times a person's heart beats per minute. 
A normal person's resting heart rate is typically 50-90 BPM. 
In some real-life scenarios, the heart rate is calculated over a much shorter period of time in order to obtain real-time heart rate changes. 
PPG (photoplethysmography) sensors on Flowtime headband use a light-based technology to acquire the heart rate, which is accurate when they have a good contact with the skin.
1. Ananya Mandal. What is Heart Rate? News-medical.(2019-06-05). <https://www.news-medical.net/health/What-is-Heart-Rate.aspx> 2. Nanchen, David. (2018). Resting heart rate: What is normal?. Heart. 104. heartjnl-2017. 10.1136/heartjnl-2017-312731. 3. Block, Frank. (1994). What is heart rate, anyway?. Journal of clinical monitoring. 10. 366-70. 10.1007/BF01618415.