The tiny variation in time between each of your heartbeats.
HRV is a beautiful indicator of your nervous system's balance. When you have high HRV, it means your body is resilient, well-recovered, and ready to take on the world. Low HRV is just your body's way of asking for a little extra rest and self-care.
Modern health trackers love this metric because it predicts how you'll handle stress. It's like a 'readiness score' for your day.