The parasympathetic nervous system is a part of the autonomic nervous system that helps the body relax and conserve energy. It is often referred to as the "rest and digest" system because it promotes functions that occur when the body is at rest, such as digestion and lowering heart rate. This system works by releasing neurotransmitters like acetylcholine, which signal various organs to perform their functions more efficiently. For example, it stimulates the salivary glands to produce saliva and encourages the stomach to digest food, helping the body recover and maintain balance after stress.