Careful examination of your material in its final format is completed at this point. Text and visual components are checked on its own or compared to a previous copy.
Looking for typos, incorrect page numbers and any other errors that may have crept in, that the previous editors have missed.
It can happen, and that is why editors will complete several passes and a proofreader might be a fresh set of eyes to catch anything that slipped through!
This is the final stage of the editing process before your work goes to print or published online.