Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats. It is measured by the variation in the beat-to-beat interval.

The "Daily Heart Rate Variability Summary" files include daily granularity recordings of your HRV during a sleep. The description for the values of each row is as follows:

rmssd: Root mean squared value of the successive differences of time interval between successive heart beats., measured during sleep.
nremhr:  Heart rate measured during non-REM sleep (i.e. light and deep sleep stages).
entropy:  Entropy quantifies randomness or disorder in a system. High entropy indicates high HRV. Entropy is measured from the histogram of time interval between successive heart beats values measured during sleep.