Places Excel in a mode where you can preview the pages in situ, allowing you to continue operating Excel almost normally. 

This is what makes it different from 'Print Preview'. You can enter data, formulas, etc.  The only difference you may notice is the size of the fonts are shrunk and you get a 'watermark' showing with the printed page number.  This is a great way of moving page breaks to ensure a print comes out exactly as expected. Excel 'zooms' the data to fit to the page after you have dragged the page breaks how you want them.

To exit this view press the 'Normal' menu option on the same 'View' menu.