While many believe that proofreading is a final edit, it is so much more. The proofreader will initially acquire your manuscript after it has gone through any previous editing you have acquired to look for mistakes since your final edit. This will be considered the ‘final read through’.
After your manuscript has gone through the design and formatting stages, the proofreader will check for consistency and errors. Some of these errors may be in spelling or punctuation, but generally it is to make sure that the stylistic choices made by the author and designer are consistent throughout the entire manuscript. Some things that the proofreader looks for is consistent page numbers, headings, font choices/indentations, spacing, etc. Beyond checking the style of the work and those style choices, the proofreader will not make any further alterations which could change the meaning of any of the text.