To save on costs and speed up the printing process, books, flyers and so on are printed on parent sheets. Because of this pages cannot be printed on the sheet following the same sequence of the acual page order, that is, page 1 is usually not printed next to page 2, even though the finished book will result in those two pages being next to each other.
Take an old magazine and detach all spreads. If the magazine has 36 pages, you will see that the cover is printed next to the back cover, page 2 is printed next to page 35, page 3 is printead next to page 34 and so on. Once the magazine is folded, cut and stiched, the pages will all fall into place, from page 1 all the way through to page 36.
The act of creating printer spreads and placing them onto parent sheets, with the purpose of creating reader’s spreads at finished product is called imposition. See also the definition of imposition on Wikipedia.