You’ve written a whole manuscript, probably countless drafts. Other people have read it and given you feedback. You’ve used that to make it every better. It’s done! Finally! But how’s your spelling? Punctuation? What’s your stance on the oxford comma? Hey, watch your tone.
Copy editing and proofreading are similar tasks done after a manuscript has been deemed complete by the author. You can do them yourself, but, like with beta readers, someone else who has never seen your manuscript before will have a better chance of catching any errors or inconsistencies.
This part of the editing process works out anything that isn’t working, spelling, grammar, fact-checking, consistency, style, and formatting. They do it all. Copy editing happens before proofreading, but together they mark the end of your long drafting and revising journey (hopefully) and you will have a finished, polished manuscript when they’re done.
Now you just have to sell it.