The Autonomic Nervous System (ANS) is part of your nervous system, which controls things you do automatically, without thinking about them. The two parts of the ANS are the Sympathetic Nervous System and the Parasympathetic Nervous System.
The Sympathetic Nervous System takes care of your body when you need energy to fight or run away from something. It makes your heart beat faster, your breathing quicker, and your muscles ready. When you're excited, your Sympathetic Nervous System is active.
The Parasympathetic Nervous System is active when you're relaxed or resting. It makes your heart rate slow down, your breathing deeper, and your muscles relaxed.
The Autonomic Nervous System is always working to keep your body balanced and running properly. Together, the Sympathetic and Parasympathetic Nervous Systems make sure you have the energy you need when you need it.