For a nonfiction book proposal, a publisher will ask for a brief synopsis of the content of each chapter in your book. The synopsis of each chapter should only be about a paragraph long. The key to a good synopsis is being very specific about the content of each chapter. Keep in mind this is all an editor or publisher has to go by to determine what your book is actually about. For example, don't say the book covers ten steps to a happy marriage, be sure you include what those ten steps are. The biggest complaint publishers have about the chapter-by-chapter synopsis is it is not specific enough.
If you are proposing a fiction book, the synopsis is not done chapter-by-chapter. Instead they want a summary of the overall plot. For more, go to: http://storysensei.blogspot.com/2006/08/whats-chapter-by-chapter-synopsis.html.