Heart rate variability (HRV) means the variation of time between subsequent heart beats measured in milliseconds (ms). Heart rate is not completely regular. For example, at rest it is normal that heart rate fluctuates in the rhythm of respiration, increasing during inspiration and decreasing during expiration. The R-spikes in ECG graph illustrate the contraction of heart's left ventricle and the time between subsequent R-spikes is called R-R interval (RRI).

More information about this can be found from Firstbeat's website here.