It's a cross-referenced introduction to everything that was cut or it otherwise missing from the Bible. Unlike an encyclopedia, the Unabridged Bible highlights what's interesting and unique. And it offers a variety of ways to start reading, including a brief summary to get you started, curious facts or quotations or whatnot to whet your appetite, and, frequently, pictures to enhance your experience.
If you encounter something you're not familiar with — a person, a place, a book, whatever — odds are you can click on it to find out more. Or if you're looking for information about a particular topic — a book missing from the Bible, say, or a particular passage in the Bible — the master index will point you in the right direction, even highlighting the information you want when you click on a link.