The hinge is the joint of the binding of a hard-back book, (where the spine meets the covers). It usually provides additional strength at the flex point. This setting in CoverBuilder is also used for the scores of a paperback book. (The area where the pages are glued to the cover, see screenshot above) The score can be visible on a paperback.
With CoverBuilder you can choose to add (+ Hinge/Scrore) or detract (- Hinge/Scrore) the hinge or score of the board size as shown in the screenshot above. CoverBuilder makes sure you never have to calculate with any values yourself!