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!
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, and so does .505.
Read the letter alphabetically, and the numbers after it as a 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!