Heart Rate Variability (HRV) is measured by looking at the time interval between successive heart beats over a specific period. That means that if you examine ten heart beats, there will be a difference in intervals between each successive beat and those differences are averaged into some number that indicates there was either a lot of variability between all of the beats or very little variability during the period the ten beats occurred.
The way HRV is captured is by a string of numbers called “RR Intervals.” The RR Intervals are measured in milliseconds. For perspective, a resting heart rate of 60 beats per minute means that there is an average of one heart beat per second. The difference between two of those beats will be a percentage of that once second, so milliseconds are the best unit of measure.
All of the complex measurements that you will encounter (Fourier transformations, High Frequency/Low Frequency, rMSSD, pNN50, etc) are all derived from this string of numbers. It can be hugely frustrating looking at academic papers and articles/blog posts without understanding that because they all dive right into these derivations and at first it looks like HRV has scores of different measurements. It is one measure with a lot of interesting interpretations. And you will develop your own favorite over time.
Find out more by reading about How To Get Your Numbers.