It is an obi, which is also known as a strip of paper looped around a book or other product. As Wikipedia continues to mention, Many books in Japan are supplied with an obi, which is normally added outside any dust jacket. [...] In English, the term belly-band is sometimes used instead. The following image is the obi on Tokyo Ghoul:re volume 7.

Only top voted, non community-wiki answers of a minimum length are eligible