A sample-and-hold circuit is an electronic device that captures and maintains a voltage level for a specific period. It samples an input signal at a particular moment and holds that value steady, allowing for accurate processing or conversion, such as in an analog-to-digital converter (ADC). This is crucial in applications where signals change rapidly, ensuring that the system can work with a stable value. The circuit typically consists of a switch, a capacitor, and an operational amplifier. When the switch is closed, the capacitor charges to the input voltage. Once the switch opens, the capacitor holds the voltage until the next sampling occurs, providing a reliable representation of the input signal over time.