UART, or Universal Asynchronous Receiver-Transmitter, is a hardware communication protocol used for serial communication between digital devices. It transmits data one bit at a time over a single wire, without requiring a clock signal, making it 'asynchronous.' UARTs are commonly found in microcontrollers, embedded systems, and IoT devices, enabling direct communication with peripherals like sensors, modems, or other computers. Security-wise, UART interfaces can be exploited if exposed, as attackers may intercept or inject data, making proper access control and physical security crucial. Understanding UART is important for hardware hacking, debugging, and secure device design.