Synchronization in digital communication
In digital communication, the transmitter sends discrete &quot;symbols&quot;1, usually at a fixed rate, called the baud rate.
The receiver must figure out when to sample the channel in order to recover the symbols correctly. The problem is that there is usually no timing reference shared by the transmitter and the receiver. The delay from the transmitter to the receiver is an unknown amount of time, and even if the receiver knows the nominal baud rate, its internal clock frequency might be slightly different from that of the transmitter.
Therefore, the receiver must be able synchronize itself (both frequency and phase) based on information found only in the signal. There are many ways to accomplish this.
For example, the normal asynchronous serial protocol (a.k.a. &quot;UART&quot; protocol) frames every data byte with fixed-value &quot;start&quot; and &quot;stop&quot; bits, so that there is a high-to-low transition at the beginning of every byte. This allows the receiver to phase-align itself and sample the individual bits. In this case, small frequency errors (a few percent) don't matter, since the phase alignment happens so often.

1 A symbol is any specific state of the channel. It could be a voltage, a current, a frequency or a phase measurement.