The human heart beats at a non-constant rate; Heart Rate Variability (HRV) is a measurement of this inconsistency.
Although HRV manifests as a function of Heart Rate, it originates in the nervous system. HRV therefore provides unique information from the information contained within Resting Heart Rate.
The autonomic nervous system, the branch of the human nervous system which controls involuntary aspects of our physiology, has two sub-branches: sympathetic (activating) and parasympathetic (deactivating). Parasympathetic stimulation reflects inputs from internal organs, like the need to digest after eating a meal, and causes a decrease in Heart Rate. Sympathetic activation is a response to stress, exercise, and disease, and causes an increase in Heart Rate. HRV emerges from the interplay between these two competing branches.
In a balanced nervous system, our hearts are constantly getting mixed messages – commands to increase Heart Rate from the sympathetic nervous system and commands to decreases Heart Rate from the parasympathetic nervous system. These mixed messages cause the resulting Heart Rate to be in a constant state of fluctuation.