A UART (Universal Asynchronous Receiver Transmitter) was one of the first really successful integrated circuits. It makes PC serial communications possible and all of Brainboxes Serial Devices (and almost all Bluetooth devices) contain a UART. In order to transmit data along a single wire it must be converted from its native parallel format (inside the PC) into a stream of 1’s and 0’s, and vice versa on the receive side.
This is exactly what the UART does. It takes a byte of data and transmits it, at a pre-determined rate (Baud rate), along a wire. It wraps the byte with a start bit and a stop bit so that the receiver can get ready to sample the incoming byte. The use of a pre-determined data rate and start and stop bits allow data to be transferred asynchronously, that is transmitter and receiver are never synchronised.