A simple overview of how to think about heart rate variability (HRV) can be found in this blog by the Oura ring creators.

The caveat of this overview is that it ignores some physiological facts about HRV, notably, that HRV can be measured mathematically in different ways, not all of them are expressed in the units of milliseconds. That means, the idea of speaking HRV increasing or decreasing does not always reflect underlying physiology well. Such simplification helps create neat info dashboards which is valuable as a start, but as the questions we are asking from HRV get more complex, so should the means we use to answer them. Welcome to the ever-evolving science of biometrics and the underlying biological oscillations they help capture!