It's essentially a book's GPS coordinates within the library. Call numbers are usually found on the spine of a book. If the spine is narrow, look on the cover on the left side.
In the catalog, look for a message like the one below that says "Available at Jamestown - Raugust Library" followed by a string of numbers and letters.
The string of numbers and letters? That's the book's call number!
TIP: It also helps to jot down or have handy the title of the book you're looking for.
For this example - 808.51 F843s - head upstairs and look for the 800s. Browse a shelf until you find books with call numbers that start with the number 808.
Read these as a decimal. So, .5 comes before .51, but so does .505.
.5 = .500
.505 = .505
.51 = .510
Read the letter alphabetically, and the numbers after it as another decimal. At this point, you can also start browsing for the title of the book you're looking for.
Take a look at what's next to the book you're looking for on the shelf. Since we organize by subject, you may find other useful books nearby!