What does "stacks" mean?
'Stacks' are the open shelves of library books. If the catalogue says a book is in the stacks, then it's on a shelf in the branch indicated in the catalogue. All books are placed on the shelf in call number order.
Once you have searched the catalog for the item and have located the call number, click on the coloured box (it is a map of the correct floor and will guide you to the call number).