Your Target Heart Rate is a pulse measurement that lets you know that you are exercising safely and effectively. As you begin walking, it is very important to know your THR - it allows you to measure your exercise intensity.
You should take your heart rate for 15 seconds once you've begun exercising for 3-5 minutes. If you are above your THR - slow it down. Step up your pace a bit if you haven't reached it by 5 minutes!
Type in your age and press the "Calculate THR" button to learn your upper and lower end Target Heart Rate for a 15 second count.