Short answer: Just hit the green sync icon.

Longer answer: When you have auto-sync enabled in the Sync pane of the Zotero preferences and you make changes to Zotero data, Zotero syncs automatically in the background. If the version of an item you changed locally is also changed on the server (i.e., from edits made on another computer), Zotero needs your help resolving the conflict. Since it would be distracting for the conflict resolution window to pop up while you were busy working, background syncs don't automatically trigger it and instead display a warning with that message.

We'll try to clarify the message in the next release.