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If you want to transmit successful information between a sender and a receiver, there must be an agreed format with data format, timing and protocol. In asynchronous communication the clock rate is set on both sides and then fixed(like on RS232, then it is called baud rate). I synchronous communication the clock is transmitted together with the data, either coded with the data(synchronous port) or as two lines(data + clock line). This is the same as on SPI. The master device supplies the clock.
The asynchronous transmission has an agreed distrance between two bit. With a startbit(always a high to low signal) the receiver can then sychronize himself since the transmission speed is defined. A typical example is the RS232 interface. Here are more details how it works: https://www.lammertbies.nl/comm/info/RS-232_specs.html .