Copyeditors review text for grammar, punctuation, usage, consistency and clarity. Their work helps create a clean manuscript that can move into book formatting. Proofreading is a later quality check on the laid-out pages, where errors can be introduced during design or conversion.
Neither stage should be treated as a substitute for the other. Schedule copyediting before typesetting and proofing after the final layout is produced. A clear sequence helps authors avoid paying to correct the same pages twice and gives the finished book a more professional reading experience.