The line idles high. A frame begins with a start bit, a single 0, whose falling edge signals the receiver that data is coming and gives it a timing reference. Then come the data bits, typically 8, sent least-significant-bit first. Optionally a parity bit follows for a basic single-bit error check (even or odd parity), though many links use none. Finally one or more stop bits, always 1, return the line to idle and guarantee a minimum gap before the next frame. Both ends must agree on every parameter, number of data bits, parity, stop bits, and the baud rate, which is why the shorthand "8N1" (8 data bits, no parity, 1 stop bit) is so common. The key conceptual point is that there's no clock line: the start bit's edge is the only synchronization the receiver gets, and it samples the rest of the frame using its own locally-generated baud timing.
Communication Protocols · Interview question
Walk through a UART frame.
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From the lesson
UART
Two wires, no clock, and a pre-agreed baud: the UART frame (start/data/parity/stop), how the receiver recovers timing by oversampling, and an interrupt-driven RX ring buffer.