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The signature of a book is not its autograph, but how the pages are folded and then cut during printing. Fold a piece of paper into eighths, then cut off the top, right, and bottom folded sides, which leaves the left outer fold. That is a signature.
When a book printing company prints your book, they print onto a large piece of film or paper with several individual pages. When this large sheet is folded, the pages fall in order. Since these signatures must have an equal number of pages divisible by 16 (except for digital, 4 is OK for digital).