Developmental editing, or structural editing, deals with the big picture, high-level questions of manuscript development.
Most developmental editing takes place in the world of fiction, and it starts with an evaluation of the manuscript as it stands. What draft is this, a rough first draft, or one that's been polished and polished, and still isn't coming out quite as shiny as the author wants? What is the overall plot, voice, theme, and structure. Or is there a discernible plot, or do things just... happen? Who is the intended audience, and what are the author's goals for the project? If self-publishing, the author is the final judge on whether the work fits their needs or not; for traditional publishing, the approach may be very different, with notes coming from a prospective acquiring editor or literary agent that most be taken into consideration. Developmental editing for fiction is a broad scope of work that can include voice, style, internal consistency, story beats, set-up and payoff, character development, themes and language choice, and many other factors that all go in to making an amazing story come to life.
The structural needs of a nonfiction book are, obviously, pretty different from fiction. But overall, the concept remains the same: Who is the audience of your book? Are you communicating to them what you hope to be communicating? Are things ordered in a logical, sensible, and consistent way throughout the book? Does your thesis or concept make sense, and is it supported and substantiated the way you wish it to be? Do you require fact-checking? Is the work too technical for your audience, or does information need to be convened through graphs and charts? Or was your work, at present, merely a good idea and an attempt at an outline? Do you need to start from the very foundations to build your work?