heart rate variability is the measure of variation in time btwn heartbeats. Apparently greater variation in the time between beats is a sign of overall health, cardio health, a body fully recovered, etc.

Things like stress, exhaustion, poor fitness cause low HRV. It fluctuates enough from day to day that a lot of people use it as an indicator of how 'stressed' or recovered they are and plan their day's workouts around it.

My understanding is most cheap heart rate monitors aren't precise enough to provide accurate HRV numbers but I may be mistaken.

Here's a Harvard primer on HRV: https://www.health.harvard.edu/blog/heart-rate-variability-new-way-track-well-2017112212789