A call number is a number assigned to a book that is tied to a physical location in the library. So the call number should tell you where in the stacks that book (or magazine, or DVD, etc) is located.
Here at Holman Library, we use the Dewey Decimal system. Most public libraries and many community college libraries also use this organization system. When looking for an item, you will see a number that might look like this: 940.534 F828xr
To use this, you will go to the stacks and look for corresponding information. So this number, would be located on the shelf that has this information on the end: