Heart Rate Variability has been studied for over 40 years. It describes the variation in the time between heart beats, so it’s a little bit different to heart rate. In simple terms gives an indication of how the external world, especially stress is impacting on the whole body. HRV is used by many athletes as an indicator of their overall body function. The availability of wearable fitness trackers means that there has been a huge amount of HRV data collection since many of these wearables send the data back to the app provider.
But there is not standard way to measure HRV. So it is an evolving science. HRV is measured in the Apple Watch 3, and Apple are very tight-lipped about how they are measuring.
For a very in-depth and informative, but quite technical discussion of HRV, see: