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 slowing the heart rate and stimulating digestion. This system works in opposition to the sympathetic nervous system, which prepares the body for stressful situations. The parasympathetic nervous system uses neurotransmitters like acetylcholine to signal various organs, helping to maintain homeostasis and support overall health.