Heart rate variability (HRV) is the measure of variation in time between each heartbeat while you’re sleeping. For example, a resting heart rate of 60 beats per minute is simply the average over that minute. However, within that given minute the time between each heartbeat can vary by 100 milliseconds (ms) or more. HRV is the measurement of that variability while you're asleep.
HRV for healthy adults will generally fall between 20-120ms. HRV is highly personalized and tends to go down as we age. That said, a higher than average HRV is a strong indicator of readiness and recovery, while a lower than average score tends to indicate elevated stress and/or physical strain