Synchronization is the precise coordination of multiple events, or mechanical devices. In computing, it refers to the coordination of hardware devices, such that the data they contain or provide is made to be identical, within an acceptably brief period of time.
Many operating systems will automatically synchronize the device's system clock with a remote "time server," whose accuracy is authoritative. This synchronization can correct minor errors in your computer's internal clock, that can compound over time to make your system clock incorrect by a margin of seconds, or even minutes.
Another example is a cloud storage service, such as Dropbox or Nextcloud, that synchronizes changes file data across multiple Internet-connected devices. When a file is changed on any synchronized device, the data is automatically updated on all of them.