Of course, we aren't talking about your autograph, but a printing concept. As you may know, we don't always print documents in the one-page-per-sheet way that your office laser jet does. Instead, we may print several pages of material on a single, larger sheet (that's called a press sheet) and then fold it and cut it to get the final finished page sizes.
What that means is that one large piece of paper coming off the press (before it's folded and trimmed) could hold four, eight or more pages of material. That large piece of paper containing multiple finished pieces is called a "signature," and the number of finished pages in one signature is called the "signature unit."
The key to properly planning your multi-page documents is to think about the signature unit. If you have a project that is nine pages long and the signature unit is eight (meaning the signature contains eight finished pages), you would use two signatures: one signature for the first eight pages, and a second signature for that last (ninth) page. But if you were to do a little bit of editing to reduce your document page length to eight pages, you would only use one signature.
By being aware of the signature unit (the number of finished pages that can fit on a press sheet) required for your project, you can remove or add content so that your final product fits the signature, which reduces waste and saves you money.