Biofeedback is a technique you can use to learn to control your body's functions, such as your heart rate. With biofeedback, you're connected to electrical sensors that help you receive information (feedback) about your body (bio). This feedback helps you focus on making subtle changes in your body, such as relaxing certain muscles, to achieve the results you want, such as reducing pain.
In essence, biofeedback gives you the power to use your thoughts to control your body, often to help with a health condition or physical performance. Biofeedback is often used as a relaxation technique.