The Parasympathetic division of the autonomic nervous system is known as the "rest and digest" division.
It is what controls the day-to-day "normal" functions of the body when we are not aroused, alarmed, or in a state of vigor such as exercise. For example, it keeps us breathing at a steady rate between 8-16 breaths per minute, keeps our heartrate between 60-90 beats per minute, keep blood flowing to our digestive organs and kidneys to process nutrients and filter out wastes, respectively. Hence the term "rest and digest."
It's antithetical system is the Sympathetic subdivision of the Autonomic Nervous System (ANS), which upon alarm or proper stimulus (activity): speeds up heartrate and respiratory rate, slows down digestive and kidneys function, and diverts blood from those organs (intestines and kidneys) to heart, lungs, and skeletal muscle for work and movement, aka "fight or flight."
