If you don't know what copyeditors do...

Or you are about to self publish and think you can't afford a professional copy editor take a look at this video produced by successful self publisher Catherine Ryan Howard.

In Catherine's book (and she's written three) there are only two "acceptable excuses" for not paying a professional copy editor.
Number One is you are already one yourself (even then I'd suggest you get someone to look over it: your eyes can all too easily see what should be on the page instead of what actually is there). 
Number Two acceptable excuse is that your book has no words in it. 
Everyone else can't afford not to pay for a professional edit.

You can read more about Catherine's adventures in self publishing on her excellent blog at http://catherineryanhoward.com