A proofreader reads through the final manuscript and corrects spelling and grammar issues. They will also check the design of the book and make sure that headings and titles are consistent.
If a book is riddled with errors it will make it hard or the audience to read and will negatively effect book reviews and sales.
The aim of a proofreader is to change as little as possible. Unlike a copy editor, they will not encourage authors to rephrase parts of the text to help make it easier to understand.
A typical proofread will include checking page numbers, making sure titles are consistent and reading through the text to make sure that there aren’t any typos or errors.
However closely an author and editor believe they have edited the work, a manuscript will always benefit from having a proofreader look at it afterwards.