Biofeedback is a fascinating subject. It involves learning to control your body with your mind. It is essentially controlling bodily processes which are usually considered to be involuntary. Some people can lower their heart rate, blood pressure, muscle tension, skin temperature, stress levels, and even pain perception through regular practice. Usually mechanical instrumentation and sensors are used to help provide the feedback, but you can measure some results like heart rate with something as simple as a watch with a second hand.
Biofeedback creates greater awareness and control of your body. It's kind of like using the feedback you get from a heart rate monitor, but the changes are initiated with your mind rather than by adjusting the intensity of exercise. When I first heard of this I was a little skeptical until I tried it. Sometimes we forget how powerful our minds are. Have you ever awakened from a nightmare sweating or with your heart pounding? There was obviously no physical exertion on the body, but the dream was perceived as real so the body responded accordingly. If you envision yourself in a fight with someone or in an intense emotional situation, your heart rate, respiration, sweat, and adrenalin may start flowing. Biofeedback is just learning how to control those things to a certain degree.
When this skill is developed, it's almost like having a superpower and being able to control the weather like Isis could. (For you younger readers, she had the same powers as Storm from X Men) Some of the reasons people learn to practice biofeedback are to control anxiety, motion sickness, high blood pressure, asthma, incontinence, migraine headaches, pain control, etc. There are now even video games and computer software programs to help people practice biofeedback.