Proofreading is the systematic examination of textual and visual elements after layout (i.e., after the material has been edited and converted to final form, and before it is published) to find errors and mark them for correction. It’s the last check point before a manuscript goes to publication.
The version of the material that I would work on is called a “proof.”
As a proofreader, I would:
check to ensure there were no syntax, grammar or spelling errors
check to ensure typesetting elements are in order, i.e., check for any issues that may have cropped up through the design/layout process such as unfortunate page breaks, headings not being aligned and incorrect images used
query any issues that fall outside of the scope of responsibility of a proofreader that I think would be important for you to note
I would not:
make any structural, stylistic or copy edits, unless previously agreed
What you may expect from me:
a fully-proofread manuscript that has been marked up using standard proofreading symbols (or some other agreed-upon method)
