One of the first things you need to know when using a heart rate monitor is what your maximum heart rate is. The standard formula used is to subtract your age from 220. You can then calculate at what percentage of maximum you should be exercising at.
When doing cardiovascular training, research says exercising at 60-70% of your maximum heart rate will be best for fat loss and to build endurance, while exercising in the 70-80% of maximum range will increase your fitness.
There are a number of features available in modern heart rate monitors. Some are more useful than others. Of course, a constant measurement of heart rate is vital. It is helpful to be able to set zone alarms or otherwise be able to see where you are compared to your maximum heart rate. Some will do all the calculations for you, telling you when to ease up or to pour it on, depending on your workout for that day. Others will just show a heart rate number, and you will need to know what the upper and lower limits are for your workout.