In order to find an item in the library you need to know what library it is in, and what the call number is.
The holding information tells you which library(s) have the item, what the call number is, and where it's located.
If the item is located in the University Library, pay close attention to the Location field. If it is located in the stacks, you can check the item out. If it is located in Reference, you cannot check the item out.
All books are shelved by Library of Congress (LC) call numbers. At the end of each row of shelves in the stacks there is a card showing which call number ranges are shelved there.