The UART stands for Universal Asynchronous Receiver/Transmitter. It is not a communication protocol itself, but rather a piece of hardware--an integrated circuit (IC) or part of a larger microcontroller--that is responsible for implementing asynchronous serial communication. Its primary job is to handle the conversion between the parallel data format used inside a computer or microcontroller and the serial data format used for transmission over a single wire.
When transmitting, the UART takes a byte of data (8 bits in parallel) from the CPU and converts it into a serial stream. It adds a start bit, an optional parity bit, and one or more stop bits to frame the data, and then sends the entire package out one bit at a time at a specific baud rate.
When receiving, the UART does the reverse. It detects the start bit, reads the incoming serial bits at the correct baud rate, checks the parity bit for errors, removes the start and stop bits, and reassembles the original byte of data, which it then makes available to the CPU in parallel form. UARTs are the fundamental hardware components that enable communication protocols like RS-232C and RS-485.