HRM for short can be a watch or a chest strap with a gizmo on it. There are many designs but they are all intended to monitor your heart rate. I use a Polar FT7 HRM with a chest strap. It lets me know in real time what my current heart rate is and how many calories I'm burning (plus some other crap). If you are serious about exercise, I highly recommend getting a HRM. This way it takes all the guessing out of how many calories you burned. I'm not going to go into details on them but they can get super fancy. Some are Bluetooth ready where they can connect to your smart phone. I purchased mine in month 2 and it made recording my burn 1000 times easier. I purchased mine at Best Buy but I've seen them at Wal-Mart, Sears, Dicks Sporting Goods, etc. You could probably get online and find a better deal then the store.
Note: I am not a doctor nor play one on TV. These are my opinions and mine only so please use my advice with caution. This might be the only blog that comes with a warning label.