Sorry but I still don't understand it. I've read about it and can read the words on the page, ie "the measure of the variation in time between heartbeats" but still don't completely get it.

If you are at rest and have a low heart rate then your HRV would be higher (as in the time between beats is higher since the heart is beating slower). If you are exercising the time between beats (HRV) goes down because the heart rate is high. A low HRV can indicate stress etc, but isn't that just another way of saying your HR is elevated? It says HRV indicates how quickly your heart recovers but isn't that still the same as how quickly your heart rate slows down?

So, I still don't understand how this metric is different than just tracking your HR.

Can someone explain like I am a child?

So, it can be confusing. It took me a while to wrap my head around it as well. But basically think of your heart as an engine in a car. You want your car engine to be very responsive to what you tell it to do. When you even tap the gas, you want it to give an appropriate response, and when you get off the gas, you want it to come back to baseline quick. If your car isn't tuned well, then when you tap the gas, perhaps the engine may rev slower than you'd like, and may not come down to baseline as quick since perhaps the air/fuel mixture isn't right and it's idling rough.

In the body you've got sympathetic tone, and parasympathetic tone. Sympathetic is your fight/flight/stress, and parasympathetic is your rest/digest. When you're working out, you want high sympathetic tone, and when you're sleeping, you want low sympathetic tone - and the spectrum between.

People who are out of shape, will force their body into high sympathetic drive (pushing on that gas pedal hard) to keep up with the demands on the body. And then when they stop doing that activity, their body still has to work hard to come down from that high demand, clean up metabolic waste, repair, etc.

People who are in shape can more effortlessly accomplish an activity, so their body's sympathetic drive doesn't have to activate as much (just tap the gas, rev it up a little, then come down to baseline). Even just standing up to walk across the room, your HR will very appropriately match your exertion, and come down quick.

If you're under a lot of stress, sick, lack of sleep, your heartrate will go up because your body is under stress and sympathetic tone is high. This will decrease your HRV as the gas pedal is sort of being pressed a little the whole time.

So basically, you want your body to be able to quickly and effortlessly expend just the right amount of heart beats you need for your body to function, and then come down to baseline. And your body is constantly doing various functions, even while sitting and watching TV, writing something, or sleeping.

At least that's how I understand it. Could be wrong.