Heart rate variability (HRV) is the physiological phenomenon of variation in the time interval between heartbeats and is measured by the variation in the beattobeat interval (commonly stated as RR interval). It is well known that HRV is directly related to the body's interdependent regulatory systems and can be a powerful measure for an athlete's recovery status.
Metrics for HRV

RMSSD
Root mean square of successive differences. This value is said to be a strong indicator for parasympathetic activity and thus large values indicate a better recovery status. 
lnRMSSD
Natural logarithm of RMSSD. The logarithm is used to achieve a better scaling. 
SDNN
Standard deviation of RR intervals 
5 minSDANN
Standard deviation of the average RR intervals calculated over 5 minutes 
pNN50
Proportion of successive RR intervals that differ by more than 50 ms 
pNN20
Proportion of successive RR intervals that differ by more than 20 ms
References
 Heart Rate Variability Explained, hrvtraining.com
 Heart Rate Variability For Training, marcoaltini.com