Usually, HRV can only be measured with specialised devices, but the Apple Watch series 4 and newer is able to use electrical and optical sensors to accurately monitor heart rate and HRV.
Heart rate variability is where the amount of time between your heartbeats fluctuates slightly. if your heart rate is highly variable, this is usually evidence that your body can adapt to many kinds of changes which is positive.
However low heart rate variability is considered a sign of current or future health problems. And can be a symptom of conditions such as:
-Diabetes
-High blood pressure
-Heart arrhythmia
-Asthma
-Anxiety
-Depression
With the Apple Watch series 4 and above, low HRV can be monitored and acted upon faster.
Health app > Browse > Heart > Heart Rate Variability