What does an editor do for you?
No one knows your book like you know your book.
This sentence is used as a reason not to hire an editor, but really it contains good truth on why you probably should.
A good editor is going to help you craft your writing so that your readers can understand your book the way you do.
Copyediting exists in three levels.
Level 1 is a technical edit. This is unobtrusive. Largely it is looking at spelling, punctuation, and grammar. Perhaps some author queries will be raised but it is unlikely to include any significant reworking of sentences.
Level 2 is a structural edit. Along with the above, your editor is more likely to pull apart sentences that just don't read well. Often these are sentences missed by the author because they know the sentence inside out.
Level 3 is a developmental edit. This level of editing might involve a longstanding relationship between the author and the editor. The manuscript will likely go through more than one pass across the editors desk. It is inclusive of the everything above but there is a higher intensity of structural editing.
Generally speaking, these are done in the opposite direction.
To use a longwinded metaphor, your novel is a little house built with shiny sea pebbles. Imagine you get a technical editor to polish up each of the little sea pebbles individually so the whole house gleams. Imagine the next thing you do is have a developmental editor come and assess your house. They get to work out several holes in your little house, tell you one little pebble would shine much better on the other side of the house, and suggest you swap out multiple other pebbles for better fitting ones. Inviting your technical editor back to repolish it all once you've done all that work would be frustrating for both you and the editor.
This series of blog posts will talk about the different things that are picked up by different levels of editing and how you will benefit from them.