What is Heart Rate Variability (HRV) ?
Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.
Heart rate variability is a reliable medical indicator that provides a wealth of physiological and pathological information. 
The higher the value is, the greater your heart rate changes. The lower, the smaller.
Heart rate variability is associated with the autonomic nervous system (ANS).
When you are not doing exercise and in a more relaxed state, you'll have a high value of HRV which means your body is easy to recover. If you are under depression or anxiety, HRV goes lower. By practicing meditation, people will be more resilient to stress and HRV will increases. 
How to understand the HRV graph?
Graph of a new meditator with low HRV average.
Graph of an experienced meditator with higher HRV average.
The graph shows how HRV changes as a result of regular deep breathing after 3 minutes.
Medically, the HRV is usually analyzed by the SDNN (the standard deviation of heartbeat NN intervals ) calculated over 24 hour period. But our algorithm, designed based on the specific application scenario, uses a 1-minute SDNN analysis of HRV which can reflect the rate of heart rate change more quickly and its standard is different from the standard values used in the medical field.
1. Walters, S.. (2015). Heart rate variability (HRV): Prognostic significance, risk factors and clinical applications. 2. Shashikant, R. & Chetankumar, P.. (2020). A Review on Impact Application of Heart Rate Variability (HRV). 10.1007/978-3-030-24322-7_1. 3. McCraty, Rollin. (2006). Emotional Stress, Positive Emotions and Psychophysiological Coherence.