Developmental editing is a thorough and in-depth review of your entire manuscript.
It examines all the elements of your writing, from individual words and sentences to overall structure and style.
In fiction, this edit will also address any issues related to plot and characterisation, point of view, and pacing.
You need to be prepared to make major changes based upon the suggestions given by the developmental editor.
According to Wikipedia, a developmental editor may guide an author (or group of authors) in conceiving the topic, planning the overall structure, and developing an outline—and may coach authors in their writing, chapter by chapter. This is true developmental editing, but not the most common way of working.
More commonly, a developmental editor is engaged only after someone (usually the publisher) decides that the authors' draft requires substantial revision and restructuring. In these cases, developmental editing is a radical form of substantive editing.
Developmental editing is generally the first step of editing to seek out once you have a completed draft of your manuscript.
Please keep an open mind to the critique you will receive.
You will also need to practice patience. It’s worth the time to ensure your book is telling the story you want, and your editor will help you stay on track.
Please remember it is a critical stage of the editing process which can help elevate your work.
Source Credits:
https://www.masterclass.com/articles/what-is-developmental-editing#what-is-developmental-editing