"Synchronization" is the method by which Logos backs up, or "synchronizes," your Logos documents and settings with the Logos servers.
This process, combined with Logos on another computer or mobile device, also allows you to work on the same documents from any location with internet access. For example, if you make a note on your laptop, you can view or edit it in Logos 5 on your desktop computer.
Sync occurs automatically, when "Use Internet" is enabled in Tools > Program Settings (recommended). You can also sync manually, by clicking the "Sync Arrows" at the top-right corner of the program, or by typing "Sync Now" in the command box.
Having trouble with Sync? See article: Troubleshooting Sync Errors