HRV is influenced by the autonomic nervous system which has two elements; parasympathetic and sympathetic.
Parasympathetic controls the “rest and digest” side of things and sympathetic controls “flight or fight”. If you are stressed then your sympathetic system is overly engaged and this manifests as a low HRV.
If I overtrain then I get a low HRV because the sympathetic system is overly engaged. Work stress and other things like poor diet will negatively impact HRV. On the other hand, rest, meditation and relaxation will cause it to increase as you engage the parasympathetic system.
A high HRV is indicative of readiness to deal with whatever is thrown at you and you can think of it as resilience which you can build (or lose) over time.
It’s a fairly personal measure and it’s generally not necessary to compare with other people as everyone is different and because many factors affect HRV but what you want to look for are trends in HRV. Upwards means you are recovering/de-stressing. Downwards trend is the opposite.
How do you measure it? Most wearables allow you to measure it. I have a garmin watch which can. Apple Watch also can do it. Most accurate approach would be to use a heart rate strap but that’s probably overkill.
Measure your HRV in the morning at the same time in the same position. After a week of readings you’ll get a baseline and can see how things change from there.
It really is a useful thing to track HRV because your body deals with a lot of daily stress and sometimes you are just not aware of it because it feels like normal or the signals are difficult to interpret.
Good luck!