What is a saddle stitch?

A saddle-stitched publication has all sides of the pages trimmed except the spine. This is to make all the outside edges even. The "creep" causes the pages to protrude differently as the pages closest to the center will protrude more than the pages on the outside. If the elements are too close to the outer edge of the page, they may be trimmed off when we trim the pages to avoid a noticeable creep.

Resource:heidelprint.com