A galley for a book is a digital file with the manuscript formatted in the way it will be printed, chapter headings, page numbers, and artwork on the inside, etc. Sometimes when a manuscript is typeset, errors are put in. So when an author is sent a galley for a book, he or she should /read/ it!
I spent today looking over Noble Imposter's galley, looking for minor errors that are easy to spot with a glance over. Tomorrow I should start rereading it thoroughly.