A near-final version of a book used for last-pass proofreading and error-catching before the final print run.
A galley proof is a formatted, typeset version of your book produced after editing is complete but before the final print run. It's your last chance to catch typos, formatting errors, misplaced page breaks, and other production issues. The term dates back to when type was set in long metal trays called galleys, and proofs were printed from those trays for review. Today, galley proofs are usually PDFs that mirror exactly how the printed book will look, page for page.
The galley proof stage is your final safety net. By this point, you've done developmental editing, line editing, and copy editing. The story is locked. The galley is about catching the small things that slipped through: the missing period on page 247, the chapter header that's in the wrong font, the widow line sitting alone at the top of a page. These details might seem minor, but they're the difference between a book that looks professional and one that doesn't.
The galley proof is for catching errors, not rewriting scenes. Major changes at this stage are expensive in traditional publishing (you'll be charged for 'author's alterations') and can introduce new errors. Content changes belong in earlier editing rounds.
Galley proofs are internal documents for catching errors. ARCs are marketing copies for generating reviews. They may look similar physically, but their purposes are completely different.
Read the galley slowly and carefully, ideally in a different format than you've been editing in. If you've been reading on screen, print it out. Fresh eyes and a different medium help you catch things you've been blind to after months of editing.
Take five pages of your current manuscript and format them as if they were a galley proof: set the margins, choose a readable font, add page numbers, and format the chapter heading. Then read those five pages out loud, word by word, marking every error you find. Count how many you catch. This exercise shows you why the galley stage exists and how much your eye skips over when reading silently.