Hrm ?

Well, if you just ride around and watch what you heart rate does, as some people do, it won't do much for you.

However, if you are working a specific plan, perhaps with a coach or using a training manual, you will need to be able to work within specific zones. Different training sessions will have different objectives and the heart rate called for will obviously vary. You will target percents based on your maximum heart rate, which is ROUGHLY 220-your age (for a male).

Example, a 20 year old has a max hr around 200 beats per minute and so if the coach says do such and such a ride at 70% of max hr, he would ride at around 140 beats per minute.

Bear in mind, this is a simplified answer and more detailed answers may follow but I think that you are just looking for the lay of the land right now???

Note: Just as important as using a monitor for hard days can be using it for easy days, to make sure that good recovery is being achieved. Sometimes it is hard to ride easily enough (just being out on the road is so darned invigorating) to be fully prepared for a torturous training session planned for the next day.

a HRM works by measuring the electrical discharge of the hearts natural pacemaker through the skin on your chest and then transmits the information to a wrist or bike mounted reciever

depending on how much you want to spend you can get really basic HRMs through to super training tools which will put you into feedback overload.

basically for training you have to know your intensity and duration and while a stopwatch will give you duration a HRM is the best way to get intensity short of super expensive power meters (the polar S710 has the ability to measure both )

as easyrider says you have to have a good plan to get the best out of a HRM.

the absolute best way to get the most effective training from a HRM is to get a proper VO2 assesment done and they will give you a report telling you what your personal zones are. Books and the like wll give you formulas to use but they are only approximations.

once you know your zones for recovery, aerobic training and threshold training you can get a coach to set up a periodised program that will allow you to work on your fitness

the whole HR zones question has been covered in more detail in some other threads and a quick search should uncover them.

otherwise just keep asking questions till they get answered