Originally Posted by MoAlpha

HRV is a great measure of sympathetic and, particularly, parasympathetic activity and is actually pretty good at predicting things like mortality in heart disease and maybe in the general population. It is also very useful for monitoring your training status and overall level of stress. I do it every morning. Problem is, no matter what anyone tells you, you can't get reliable data with an optical system.

In general, the higher your HRV, the healthier (and younger) you are. However, HRV is always greater the slower the HR, so there's a confound there. Unfortunately, no app I've seen controls for rate.