Call numbers are used to arrange the books on the library's shelves so every book has a specific location. If you know how to read them you'll be able to find any book the library has.
Here's a breakdown of the call number RA643 .W37 2009
Here's how call numbers look in the library catalog:
And, here's how they look on the shelf:
Browsing is a great way to find books. The books in the library are arranged so that books on similar topics are nearby each other. If you get the call number for one book that covers your topic, there’s a good chance you’ll find more on the shelf nearby it.
Modified from: http://cooklibrary.towson.edu/help-guides/understanding-call-numbers