Biofeedback is using the signs of your body to treat yourself and to keep yourself healthy. Biofeedback is used in many professions including physical therapy, psychologists, and specialists in many different fields. Using biofeedback on yourself may include little things like taking your temperature or getting on the scale. The feed back is the reading of the instruments and your actions may be determined by what you read. If you have a fever you might put a cool rag on your head or take a fever reducer.

Some professionals use biofeedback machines that monitor and report on internal bodily functions. Generally these machines are used as second opinions and not as diagnostic tools.

Is it helpful? I believe that any system that promotes self awareness of the body and encourages people to to understand how the environment or stress affects them is a good thing. Biofeedback tends to be noninvasive but giving good information which helps reduce the stress of testing which is also a good thing.

Biofeedback is a process where the patient's brainwaves, heart function, muscle activity, breathing patterns, and skin condition are measured on a special machine, and then fed back as an overall picture which may enable the patient to make physiological changes side by side with emotional changes. Biofeedback is helpful for treating autism, asthma, learning disabilities, depression, and high blood pressure.

Source: http://www.therapies4all.com/biofeedback.html