Copyediting is professional help, focussing on the detail of the text to make it ready for publication:
Ensuring that it's clear, consistent, correct and complete.
Agreeing final content, making sure it reads well for its intended purpose
Applying consistency to the language and formatting.
Once a text has been copyedited, it is ready for the next stage - design and layout.
What is the difference between Copyediting and Proofreading?
There is often confusion between copyediting and proofreading:
Copyeditors get the raw material into shape for publication: they edit copy. When they have finished, and the layout has been created by the designer and/or typesetter, the work needs to be ready to go. It's like putting on a play: these stages are the rehearsals.
Proofreaders do the quality check and final tidy-up: they read proofs. They pick up anything the copyeditor missed and check that nothing went wrong at the design/typesetting stage. Proofreading is the last stage.