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.