storage space in a library consisting of an extensive arrangement of bookshelves where most of the books are stored
"The university's new stacks occupy three floors and hold over two million volumes."
plural of stack
"The attic was filled with old boxes, piles of newspapers, and stacks of books waiting to be sorted."
Third-person singular simple present indicative form of stack
"The librarian stacks the magazines on the shelf every evening."
In plain English: To stack something means to pile it up neatly on top of other things.
"We need to stack these boxes before we move them out."