Nerves are your body’s sensing and control system. They are formed from long chains of neurons, the same cells that make up your brain. Some nerves pass down your spinal column, but others exit the brain directly into parts of the body; these are called cranial nerves, of which your vagus nerve is the longest and most complex.
Parts of your nervous system are responsible for voluntary control, such as the moving of an arm. But other parts are responsible for controlling involuntary functions, such as the heart beating. Medics refer to the voluntary networks as the somatic nervous system (SNS) and involuntary networks as the autonomic nervous system (ANS).
The ANS is split into two branches, the sympathetic branch and parasympathetic branch, which work in opposition to one another acting as an accelerator and a brake to shift the body between states of excitation and relaxation. The vagus nerve is part of the braking system -- the parasympathetic branch. It attempts to slow your body down and promote relaxation and healing.