Synapses are crucial junctions between neurons that facilitate the transmission of signals via neurotransmitters. They allow for communication across the nervous system, enabling the coordination of reflexes, sensory input, and motor outputs. Through synaptic plasticity, they also play a vital role in learning and memory by strengthening or weakening connections based on activity levels. This adaptability makes synapses essential for overall brain function and the body's ability to respond to stimuli.