Copy editing is two separate words, because it is the combination of two different processes, the "copy" (the material) and the "editing" (the process of improving the material).
Copy editing involves a range of activities, such as:
• Examining your text for grammar, punctuation and other mistakes
• Ensuring consistency in spelling, style and language use
• Detecting and correcting errors in sentence structure and diction
• Rewriting and restructuring awkward or illogical sentences
• Polishing the grammar and syntax of your text
• Ensuring that the tone is consistent and appropriate
• Enhancing the overall readability of your document
Copy editing is an important part of the writing process, ensuring that your work looks professional and polished. To learn more about copy editing, you can read this article on copy editing basics: https://www.scribendi.com/advice/copy_editing_basics.en.html.