Most first-time authors think editing means fixing commas, correcting typos, and making sure the grammar is sound. That is copyediting.
Before you can worry about the commas, you have to worry about the structure. That is developmental editing.
The skeleton of the story
A developmental editor looks at the book from 10,000 feet. They are asking fundamental questions about the narrative:
The editorial letter
When a developmental edit is finished, you do not just get a marked-up manuscript. You get an editorial letter.
This is a comprehensive document (often 5 to 15 pages long) that breaks down what is working, what isn't, and specific suggestions on how to fix it.
Receiving a developmental edit can be overwhelming. Read the letter, put it away for a few days, and then come back to it when the initial sting has worn off.
It is the hardest part of the editing process, but it is also the part where the book is truly made.