Can you explain what hrv means and how do you check it? is it true that it's not as good if your hr is perfectly steady like a metronome?

Sort of: While it is true that some studies have linked higher heart rate variability (HRV) to a higher level of fitness and overall health, the correlation is not 100%. A stronger correlation is resting heart rate. In either event, most commercial grade heart rate monitors you can purchase at your local sporting goods store or electronics store will measure HRV. Hope this helps!