Essentially it is how to read your own body and know how your body works, rather than relying on doctors. There is training to do this, and even therapies and forms of treatment for various ailments that rely on biofeedback. Doctors everywhere rely on biofeedback machines, devices we humans build to help us, machines for knowing out heart rate in neat little blips, machines for knowing our blood sugar levels, machines for seeing our innards and our bones, machines for everything, making us more and more in tune with our bodies and also reliant upon them. You've used biofeedback too, you've taken a temperature, you have looked on a scale to see what you weigh. It is also a form of alternative medicine, in that people train themselves to know responses to activities.
How this works is machines that monitor your condition and report them back to you in real time, like your blood pressure, heart rate, skin temperature, your sweat and your muscle tension. The though is that by being able to see what it is on a machine you can train yourself to understand the feelings you receive - this is biofeedback at work.
An interesting result of someone being able to know their body's systems is that they can end up controlling more muscles than were previously thought to be controllable - they can see what happens and are able to gain control over such things as their own heart, and also problems with ADHD, and muscle tension. Being able to directly control hypertension has become something that has spawned great interest - as it may be able to eventually help prevent heart attack and heart disease. Biofeedback monitoring is done through several devices, such as skin temperature measuring tools, sweat glands monitors, and Electroencephalography (EEG) machines which monitor brain activity.