By synchronising a workspace you put all files back to a previous changeset. A typical situation where this might be useful is if you find a bug and want to know when it was introduced. You might then synchronise back 10 changes and see if the bug still exists. By doing this multiple times you can isolate the exact change which introduced the bug.
To synchonise a workspace back to a previous changeset right-click the workspace and select 'Synchronise'. This will launch the 'Synchronise Workspace' dialog. Alternatively, you can find the changeset you want to synchronise back to under Submitted Changesets and select 'Synchronise to Changeset'.
Check the 'To the revisions current when a Changeset was submitted' button and press the'...' button. This will launch the 'Changeset Selection' dialog.
The 'To the revisions current at a specific Date/Time' option will synchonise to the last changeset submitted prior to this date and time.
You can see which changeset the workspace has been synchronised to within the submitted changesets list. All changesets which have not been synchronised will appear with a warning icon.