Saddle Stitching refers to a very popular book binding method in which folded sheets are gathered together one inside the other and then stapled through the fold line with wire staples.  The staples pass through the folded crease from the outside and are clinched between the centermost pages.  Two staples are commonly used but larger books may require more staples along the spine.

WHY IS IT CALLED SADDLE STITCHING?

Saddle Stitching may sound like an odd name for a book binding process that places wire staples through sheets of paper, but in the printing industry stapling is commonly called "stitching". Also, the collated sheets are draped over a Saddle-like apparatus during the stapling/stitching process, hence the name Saddle Stitching.