Part I introduces AppleScript. What is it? How does it work? Where can I use it? What can I do with it? These are the sorts of questions this part answers.
If you already have a notion of what AppleScript is and just want to get on with studying the language, you can skip the first two chapters; but you should read Chapter 3, because it contains fundamental information and definitions that are not repeated later, and on which the rest of the book depends.