HRV is the measure of the variation among the intervals of your heartbeats, because your heat does not beat at exact intervals.
Why is important?
The value for HRV, typical values between 20-100, can indicate your overall health and fitness, and also tell you how ready you are for future strenuous exercise. A higher number indicates that your body is very healthy and ready for more challenging workouts.
What factors influence it?
The amount of sleep, amount of training, stress, dehydration, smoking, alcohol, and if you about to get sick, are all things that can influence your HRV.
How to use HRV to better your fitness.
The best way to use HRV is let the value, taken in the morning, help determine how hard you train that day. If you have a lower value, you might want to take it easy or even take a recovery day to rest your body from potentially over-training or potentially getting sick.