When you enter phrases into the Google Books Ngram Viewer, it displays a graph showing how those phrases have occurred in a corpus of books (e.g., "British English", "English Fiction", "French") over the selected years. Let's look at a sample graph:
(Click on the image to see a larger version.)
John Reid has published an interesting article on his blog about the use of the Google Books Ngram Viewer to find the popularity of certain words throughout history. I don't think this will help you find ancestors, but it will display some interesting insights into history and also to the language of our ancestors.
You can read John's interesting article at http://anglo-celtic-connections.blogspot.com/2010/12/trends-in-use-of-genealogy-related.html
You can research the words of your choice at http://ngrams.googlelabs.com
