In serial communications, where each bit of the message is transmitted in sequence, stop bits are extra "1" bits which follow the data and any parity bit. They mark the end of a unit of transmission (normally a byte or character).
For example, characters on an EIA-232 serial line may have one or two stop bits added. Some UARTs even allow for 1.5 stop bits but one is probably the most commonly used. A serial connection may be described as, for example, "8N1" which means eight data bits, no parity and one stop bit.