Bleed

Printing machines cut stacks of paper, and no cutter is perfect to the thousandth of an inch. If your background colour stopped exactly at the trim line, a cut that landed a hair outside it would leave a thin white stripe along the edge of the book. Bleed solves that by asking you to extend the artwork past where the cut will happen, so the cut always lands inside the ink.

In practice this means your cover file is larger than your finished book. A full wrap bleeds on all four outer edges, so a cover file is 0.25 inches wider and 0.25 inches taller than the same cover measured at trim.

Interior pages need bleed too, but only if something on them touches the edge — a page of body text does not — and their rule is not the cover's. KDP bleeds an interior page on the top, the bottom and the outside edge only, never the gutter, because the gutter edge is bound rather than cut. So a bled 6 × 9 interior is a 6.125 × 9.25 inch page: a quarter inch taller, but only an eighth wider. A 6.25 × 9.25 page is the commonest way to get this wrong, and our KDP PDF Checker names it when it sees one.

Everything in the bleed region will be cut off. Nothing you need to survive should live there, which is the flip side of the safe area rule at the other end of the page.