Syncing your Zotero Account (i.e. your desktop piece, with your account on the Zotero website), allows you to work from different computers and ensure that your citations are backed up to your account on the Zotero website.
How do you sync your Zotero account?
Click on the Edit tab in your Zotero account and select Preferences.
Click on the Sync tab in your Zotero desktop piece, and enter the Zotero credentials that you created when you registered for an account at zotero.org.