Do you remember the auto-hide Ribbon feature in Excel 2010 that will show and hide the Ribbon (toolbar) of Excel to provide more space to use?
Well, this feature is changed in Office 2013.
Now, the Full Screen Mode feature is introduced in this new version.
If you look in the top-right corner of your Excel workbook, you will see the second icon next to the help icon called Full Screen Mode.
If you click on this, it will hide the Ribbon control and show three dots (...).
To show the ribbon again, you can click again on the dots.
And to return to full screen mode, just click on "Exit Full Screen Mode"; see: