WHAT IS EDITING?
I think the best way to answer this is to quote the masters of the craft ...
"Editing involves carefully reviewing material before it is published and suggesting or making changes to correct or improve it. The editor must communicate clearly and tactfully with all team members, and clearly mark and convey changes, suggestions, and directions. In all cases, the editor should strive to make all changes without altering intended meaning or introducing errors.
The editor should also be aware of the legal and ethical dimensions of the publishing process, including issues involving copyright, plagiarism, libel, privacy protection, and confidentiality, and the need to address biased, non-inclusive, and offensive material."
https://www.editors.ca/hire/definitions-editorial-skills
The romantic myth of an author sitting alone in their room and emerging with a perfectly polished book is just that: a myth. While writing itself may be a solo activity, the best books are products of collaboration — and professional book editors are crucial partners, as they have the knowledge, talent, and revision skills to transform your book from a rough draft to a bestseller!
From developmental editors, who give advice on story and structure, to copy editors and proofreaders, who hone the mechanics of your work, a strong team of literary editors is vital to your book’s success.