Clicking Sync will compare the local string tables with an external spreadsheet. Differences will be shown in the Sync window. There you can see each difference and choose between the left side value (your local value) and the right side value (external value). Once all the choices are made for each difference, click Save and Upload.
If you are syncing to a Google Spreadsheet clicking Save and Upload will save the local copy of your string tables and also update the external spreadsheet.
Example: Here a new language has been detected, choose the left or right radio button to keep or discard the language.
Sync Action Buttons control all choices. Select All Right or All Left to set all choices. Select Clear Selections to clear all selections.