The "STACKS" refers to a location within the library where a book is shelved. When the library catalogue says that the book you are looking for is located in the "STACKS", it means that the book is located on either the 2nd or 3rd floor of the library depending on its call number.
Anything with a Call Number:
A-K is located on the 2nd floor
L-Z is located on the 3rd floor
This book is located on the 2nd floor of the library (HD is located on the 2nd floor of the library because anything with a call number between A-K is located on the 2nd Floor).
This book is located on the 3rd floor of the library (RT is located on the 3rd floor of the library because anything with a call number between L-Z is located on the 3rd floor).