Proofreading occurs when copyediting is finished and you have proofs in hand! A proof is a copy of your work that has already been edited and formatted.
Proofreading is the last step in the editing process, where the editor makes sure everything is perfect: from headings, page numbers, and special figures to double checking where words fall on the page. The only changes made are to egregious grammatical errors, spelling errors, and other issues that can pop up during the formatting process.
Files for proofreading should be already formatted on hard copy, or in Adobe Acrobat.
If you aren't sure if you need a copyedit or proofread, send an email to firstname.lastname@example.org for clarification!