A vestibule is a pocket of covered space created by the oversized rainfly in front of the tent door. You stake it out and, as you crawl into the tent through the zippered door, you leave your wet, muddy or dirty boots behind. The boots or whatever will be protected from the weather and you don't track dirt, sticks, leaves, etc into the interior of the tent.
To get a visual on this go to
www.campmor.com
and check out your tent there.