From the app description:
X-Ray for Books - see the "bones of the book." X-Ray helps you learn more about notable characters, places, and phrases with descriptions from Shelfari.com and Wikipedia.X-Ray, which is a major selling point of Amazon's own Kindle devices, was previously introduced as a textbook-only feature with the 3.4 update for Kindle for iOS.
The feature is backed by Wikipedia and Shelfari, which is described as Amazon's community-powered encyclopedia for book lovers. iOS 5 is required for the 3.5 update, which can be downloaded from the App Store.