Proofreading is a read for mistakes, not for meaning, done slowly and with fresh eyes. The proofreader compares the final text and layout against what it should be and corrects whatever is wrong: misspelled words, missing or doubled words, wrong punctuation, inconsistent capitalization, broken cross-references and similar slips.
The name comes from "proofs," the trial printed pages that were once checked before a full print run. Today, most proofreading happens on a PDF, but the idea is unchanged. You are checking that the finished product matches the approved text, and that nothing has gone wrong in the process of turning a manuscript into a book.
It is a short, focused job. The proofreader does not restructure chapters, rewrite sentences or debate your style. That discipline is exactly why proofreading costs less than any other level of editing.
Proofreaders work against a reference, usually your style sheet from the copyedit or a guide such as the Chicago Manual of Style. Where something is a matter of taste and not an error, they leave it alone or ask you.