This is the nuts-and-bolts edit.
I check that grammar/syntax/spelling/punctuation are correct, look for any discrepancies in the manuscript (page 4 says Mandy always has lunch at Rita’s Cafe, page 50 mentions Rita’s Cantina). Sometimes called proof-reading*, it’s the final polish on an otherwise completed manuscript.
This is likely the final step before you get the book ready for publication.
(Easy Math for Writers: $300 per 50K words)