Stacks is library term for shelves of books. Libraries keep their books in the stacks. Some libraries have closed stacks, which means you can't just wander through them. Most of Trent's library stacks are open to the public. The Archives have closed stacks.

If TOPCAT lists an item's location as stacks, that means it's on the regular shelves on the 3rd or 4th Floor, under its call number.