Copy editing is a necessary aspect of publishing. Having your work copy edited means that an editor will go through the whole project, line by line, and make sure the it doesn’t have any grammatical issues, that the sentence structures are correct, that you’ve made all the right choices with regard to word usage, and that there aren’t any glaring inconsistencies. (For instance, a character doesn’t have green eyes in one chapter and blue in the next.)
While there may be notes on the overall structure, the main focus is on line level accuracy. Copy editing is necessary because as the author of the project, you can’t always see the small stuff, because you’re focused on the big picture. An editor is trained to see the line level issues, such as comma splices and incomplete sentences, that may sound right to the author, but don’t work well on the page. They’ll also provide feedback on word choice and sentence structure that may help with clarity and flow.
It’s important to remember: this is your work. You never have to keep any of the changes an editor suggests. The feedback is to make your work stronger, but the bottom line is that you are in control of your project.