With fitness trackers becoming the norm, individuals and especially athletes are looking at more matrics then ever. The standard has always been Heart Rate (at rest, at max and threshold). Now we have a new metric that the average athlete and non-athlete alike are looking at, it's called Heart Rate Variability (HRV). Heart Rate Variability (HRV) therefore is a measure of the variation in the time interval between heartbeats. Apple Watch calculates HRV by using the standard deviation of these beat-to-beat intervals measured by the heart rate sensor (also known as SDNN). The question remains...what does this mean and do you want a higher or lower number. Then, what do we do with this number?
The "Cliff Notes" answer to this is how it is tracked will determine what the number is, but in general, the higher the number the better. If the most advanced technology to measure is used (RMSSD) the higher the number the better, but what you use to track this will vary. For example, the Apple Watch uses a methodology of SDNN. This approach is/was the standard model used in the medical community to measure for mortality risks. In this case, the number will be lower then RMSSD, but still the same applies, the higher the number, the better.
Bottom line, The better the number the more prepared your body is for the next workout. Elite athletes are in the range of 50-70. Worse the number, the sign of overtraining, not enough rest has occurred, increase stress, illness or poor cardiovascular health.