The stacks of a library are the book storage area, row upon row of bookshelves tightly packed together.
Here is a hierarchical arrangement of all the books on English Wikibooks, managed in a mostly (though not entirely) automated way. At the top level, our stacks are arranged into departments — humanities, social sciences, mathematics, and so on. All the sectional units of the hierarchy below that level are called shelves.
At the top of the hierarchy is our floor plan (so to speak) showing all the departments:
For the most part, the pages in these stacks should detect when they need some sort of maintenance by a human contributor, and provide tools to assist in the tasks likely to be relevant.
This is the most recent form of a Wikibooks cataloging tradition that has been through several incarnations stretching back to the earliest years of the project.