Proofreading is the final step of the process before sending your manuscript for print. At this point, it should be free of most grammar and spelling mistakes. The purpose of proofreading is to catch any last-minute mistakes that both the author and the previous editors have missed. Since the same editor may perform both the copy and line edits, it’s important to have one more person read over the document for last-minute errors.
I want to reiterate that the person doing your line and copy edits should not be the same person proofreading your manuscript. In his poem, “An Essay on Criticism, Part II”, Alexander Pope said, “To err is human; to forgive, divine.” That applies here. You hire a proofreader because both you and your editor are human and the human mind has a particular way of categorizing things. Once the brain has accepted something as finished, it devotes resources to other things.