Bleed
Extra art that runs past the edge of the page, so a slightly off cut never leaves a white sliver.
Bleed is the part of your design that extends beyond the trim line, usually 0.125 inch on every outside edge. It gets cut off when the book is trimmed.
Books are printed on large sheets and cut down to size, and every cut can shift a hair. If your art stops exactly at the edge, that shift can leave a thin white line. With bleed, the cut lands on color instead.