Paired with your Forerunner 620 (sold separately), Garmin's Run Heart Rate Monitor gives you invaluable run data, allowing you to go faster, farther, and become more efficient than ever. The HRM Run records your cadence (the number of steps per minute), vertical oscillation (the bounce in your stride), and ground contact time (the amount of time your foot is in contact with the ground per step). Oh yeah, you also get heart rate. Don't just measure miles, measure what counts with Garmin.
ANT+ is a wireless technology that allows your monitoring device to seamlessly communicate with other ANT+ products such as speed sensors, heart rate monitors, power meters, and foot pods that track running/walking progress.