If you have a designer using InDesign that has a good understanding of how spreads, signatures and imposition software work, you are likely to save time and money when it comes time work with a printer.
There are essentially two types of spreads in the printing world; reader spreads and printer spreads. When you open a magazine that's stapled in the center, you will notice page two is opposite page three. This is an example of a reader's spread: what the reader sees. The two pages are not part of the same piece of paper but they appear across from one another. 
If you take the staples out of the magazine, you'll see that page two is actually connected to another page at the back of the magazine. This is called a printer spread; it's what a printer prints. When the magazine was printed, these pages were printed next to each other, folded and then stapled so that you received a magazine bound in the center.
If a designer provides a printer with reader spreads, the printer will have to manually change the page order to printer spreads. This will cost time and money and will increase the chances of having a problem with the project. Designers should always provide printer's spreads to a printer. Keep in mind that, for saddle-stitched jobs, your pages should be in increments of four. If not, you may end up with some blank pages in the back of your project.
A signature refers to the group of pages that are printed on the same sheet of paper. The paper is then cut and trimmed down to the finished page size. The number of pages on a signature depends on your page size and the size of the printer's sheet or roll of paper.
Imposition refers to the placement and direction of pages in a signature. Some pages may appear upside down or backwards but, once it's folded and cut, the pages will be in their proper position and sequence. A printer would set up a signature's imposition. Printers with InDesign use sophisticated imposition software such as the InDesign Imposition Plug-in to arrange the pages as they wish.
