When the user selects the Auto-fill option in their user Preferences, the timesheet will automatically include any Project / Task entries that had time reported for the preceding time period. The hours and comments will be blank, but the entries will be displayed on the current periods timesheet (saving you some typing). This may be useful if you charge time to the same projects period after period.
Note: If auto-fill is enabled, you cannot remove a project on your current timesheet that was carried forward from last period. You may attempt to remove it, however, the next time you access your timesheet, the entry will auto-fill in again. For this reason, it is not uncommon for a user to enable auto-fill at the beginning of a time period -- and after recording time for the desired projects during your first access to a timesheet, subsequently disabling the auto-fill feature for the remainder of that time period.
For additional information about timesheets, check out Working with Timesheets.