In short
Heart rate variability (HRV) is the variation in time between consecutive heartbeats, used as a non-invasive indicator of autonomic nervous system activity and balance between its sympathetic and parasympathetic branches.
A healthy heart does not beat at perfectly even intervals — the time between beats fluctuates slightly from one beat to the next, and this fluctuation is influenced by the ongoing tug-of-war between the sympathetic nervous system (which speeds the heart up, the "fight or flight" branch) and the parasympathetic nervous system, primarily via the vagus nerve (which slows it down, the "rest and digest" branch). Higher HRV generally reflects greater parasympathetic influence and a more adaptable autonomic nervous system; lower HRV reflects relatively more sympathetic dominance or reduced autonomic flexibility.
HRV is measured from an ECG or a well-validated pulse-based sensor (as used in some consumer wearables) using metrics like RMSSD (root mean square of successive differences) or SDNN (standard deviation of NN intervals), typically averaged over a short recording or overnight sleep. Lower HRV has been associated in clinical research with higher cardiovascular risk, and it is used clinically as one prognostic marker after events like myocardial infarction.
In consumer longevity and fitness tracking, HRV is popularly used as a proxy for recovery status and training readiness, based on the idea that unresolved physical or psychological stress lowers HRV the following morning. This day-to-day tracking use has some supporting research in athletic populations but is less rigorously validated than HRV's clinical prognostic role, and consumer wearable HRV values are not directly comparable to clinical ECG-derived HRV or across different device brands, since measurement methods and algorithms differ.