First, let’s set the record straight: editors don’t change your story or ruin your voice—at least, they shouldn’t. In general, authors and editors appreciate and deeply respect each other’s work; we’re on the same side and work toward the same goal. An editor’s job is to make you look good.
What editors do can depend on the type of editing they provide, but at a high level, editors improve manuscripts and other types of writing (articles, web copy, blogs, academic writing) by:
helping with structure, content, and organization
improving clarity, style, flow, and readability
enhancing word choice and sentence structure
correcting grammar, punctuation, and spelling
ensuring consistency and adherence to style guides