A call number is the number on the label on the spine of the book that helps you locate the book on the Library shelves. It looks like this in Scout:
G465 .B32 1991
And like this on the book:
G465
.B32
1991
When you find a book or other item you want in Scout, write down the call number and use the tab "Physically locate a book or other item" here to learn where in the Library those call numbers are located. You can then find the item on the shelf.