Proofreading is for last-minute touch-ups before publishing.

Some things a proofreader might look for are errors in punctuation, grammar, spelling, and simple formatting. They’ll make sure all the cross-references that say, “See page X for more information about this,” actually line up and have the correct page number. They’ll make sure a chapter number isn’t skipped or duplicated, that words aren’t cut off by graphics, that the pages look nice and clean. All the little details that you don’t really want to take the time to check are important, that’s what a proofreader will do for you—in addition to checking for typos, punctuation errors, and inconsistent grammar.