To reprint something is to publish it again, or issue it in a new form. When a book is a best seller, its publisher will reprint thousands, or even millions, of copies.
Sometimes books go out of print for a long time before a publisher reprints them, and in other cases they are continuously popular enough that a publishing company will reprint them constantly. You can call a newer printed edition of a book or magazine article a reprint, too. Reprint adds the "again" prefix re-, to print, from the Old French preinte, "impression."