I’m not sure why they call them first pass pages, since this is the last time I get to go through the manuscript.

First pass pages are also called galley proofs, or the proof.  At this point, your manuscript has been through edits (larger content changes), and line edits (nitty gritty grammar fixes and the like.)  The manuscript has been typeset and looks exactly as it will look in the published book.

A proofreader has been through it to check for line breaks and widows, as well as any other errors that have been missed.  Your editor also goes through it one more time.