It's a polar ft7 heart rate monitor. Just wear it while working out. I only got it a week before I sprained my ankle so not much use under my belt. I recommend it though. I didn't realize there was such variation in my workouts. Is it a must have probably not, but I got it on amazon for $67. There's a chest strap with a transmitor tht sends data to the watch. It hasn't bothered me at all. Especially if you already wear a sports bra.