Often I’m asked to “proofread” a piece of writing. But is that really what the person means? Usually not. Here’s the difference:
“Copyediting” refers to working on an original manuscript before it is typeset.
“Proofreading” refers to working on proof pages, which means pages that have been typeset.
So you should make sure to have your manuscript copyedited before submitting it to an agent or publisher. Later you should be given the opportunity to review typeset proof pages to look for any last-minute corrections that need to be made.