Heart rate variability (HRV) is the variation in time between your heartbeats. Not your heart rate but the gaps between beats.

A high HRV generally signals that your autonomic nervous system is functioning well: your body can shift fluidly between stress and recovery. A chronically low HRV is increasingly associated with cardiovascular risk, autonomic dysfunction, sleep disorders, and poor recovery from physical stress.

In short: it's one of the few metrics your body generates passively that gives real clinical signal, not just fitness data.