X-Ray is a feature that helps you to get to the depth of the book you are reading and justifies Amazon’s saying that X-ray takes you to the bones of a book. Get to access information from Wikipedia and Shelfari, which is Amazon's encyclopedia created especially for book lovers. But do not get excited too much as X-Ray feature is not available in all Kindle books. If your book does not support X-Ray feature, the X-Ray button of the book is grayed out. And this feature is available only for books that are in English.
So, if you want a Kindle with X-ray features, go for Amazon Kindle Touch or models after that. The Kindle Fire tablets, the Kindle mobile apps, Fire Phone and Fire TV have this app.