Structural editing assesses the manuscript as a whole and checks that the content, structure, language and presentation meet the publication’s purpose and readers’ expectations. At this stage, restructuring and rewording may be carried out to aid accessibility, clarity, style and tone, and to tighten the reader’s focus.
A structural edit might find inconsistencies in characterisation, holes in a plot, errors of logic, structural problems, or the like, so may contain substantial changes. For this reason, please ask your editor to use track changes in Word to you can see what has been edited and whether you agree with those changes or not.
As you can imagine, this may be a lengthy process, particularly if the editor identifies flaws that require you to rewrite sections.
It’s important to understand the boundary between author and editor here: in most instances, it is not the editor’s job to actually do the rewriting. It’s your manuscript, so unless you have agreed that the editor will take a strong writing role, that work comes back to you.