The Need For Synchronization
Whenever an electronic device transmits digital (and sometimes analogue) data to another electronic device, there must be a certain rhythm established between the two devices, i.e., the receiving device must have some way of knowing, within the context of the fluctuating signal that it's receiving, where each unit of data begins and where it ends.
For example, a television transmitter produces a continuous stream of data in which each horizontal line of image must be distinguishable from the preceding and succeeding lines, so that a TV will be able to distinguish between them upon reception.
Or, a serial data signal between two PCs must have individual bits and bytes that the receiving PC can distinguish. If it doesn't, then the receiving PC can't tell where one byte or bit ends and the next one begins.
So the signal must be synchronized in a way that the receiver can distinguish the bits and bytes as the transmitter intends them to be distinguished.
Read more about this topic: Comparison Of Synchronous And Asynchronous Signalling