Copyediting is the technical, line-by-line correction of a manuscript after the content is settled. It sits after developmental and line editing and before proofreading, because it assumes the text is more or less in its final form. Your copyeditor works with the words you have, fixes what is wrong and queries you about anything that might be a deliberate choice.
Think of copyediting as quality control for the reader's trust. Readers forgive an unusual plot far more easily than a book full of small slips. Every inconsistent spelling or wrong word quietly tells them the author did not care, and that is the last impression you want to make with a first book. A clean copyedit lets your ideas, characters and voice do the talking.
A copyedit also protects your style. Rather than imposing a house voice, the editor learns how you write and enforces your own choices consistently. If you prefer a single spelling for an invented place name or a deliberately unusual phrase, the copyeditor records it and applies it every time.
What does that look like on the page? A copyeditor changes "its" to "it's" where needed, spots that the narrator says "Tuesday" in one paragraph and "Wednesday" in the next, and notices that "email" appears as "e-mail" in a third of the chapters. None of these is dramatic. Together, they decide whether your book feels finished.