The document_title_parts filter allows plugins to modify or completely replace the page title contents. In this case, the code of the function that we associated with the filter hook receives the title that WordPress intends to display as an array. It then proceeds to check whether the item that is being prepared for display is a Book Review and whether or not it is a single item. While the first condition is something obvious to check for, the is_single verification is done to make sure that the code does not try to add a book author to the Book Reviews archive listing page.