An action potential is a rapid electrical signal that travels along the membrane of a neuron or muscle cell. It occurs when the cell's membrane potential changes, allowing sodium ions to flow into the cell, making it more positive. This change triggers a wave of depolarization that moves along the cell, allowing communication between cells. Once the action potential reaches its peak, potassium ions flow out of the cell, restoring the membrane's resting potential. This process is essential for transmitting signals in the nervous system and for muscle contractions, enabling functions like movement and reflexes.