Each heartbeat pushes a little more blood into the skin, which changes its colour by a fraction of a percent. Your GPU blurs every camera frame down to a coarse grid, keeps only the colour changes that repeat 42 to 180 times a minute, multiplies them about 60 times and adds them back, so the flush becomes visible (Eulerian video magnification, Wu et al. 2012). The heart-rate number comes from the average colour of your face.
Sit still in steady, bright light, facing the camera, for 10 to 20 seconds. Daylight or a lamp beats a flickering ceiling light.
A curiosity, not a medical device. The number can be wrong, and confidently so. Nothing leaves your device.