A copy edit is a general overhaul designed to establish the authenticity & consistency of the world you've created. It makes adjustments directly onto your script while maintaining your voice and style, and comes at the end of your draft process. Copy editing is designed to highlight any extraneous language, weak characterisation and dubious settings as well as your choice of words & the structure of your sentences.
A FEW MISTAKES HERE AND THERE DON'T MATTER...
I'd worked for weeks on my opening chapter. I'd got all my ducks in a row. My setting, my two main characters and the flash point. I thought it was really clear. Jac immediately pulled out one word. WAS. It should have read WASN'T. Honestly, I'd read that opening paragraph 500 times. I can't believe I missed it. One word and nothing made sense.