To be useful, every language must have a platform or standard library or API of functions for performing things like basic input and output. The core JavaScript language defines a minimal API for working with text, arrays, dates and regular expressions but does not include any input or output functionality. Input and output(as well as more sophisticated features, such as networking, storage, and graphics) are the responsibility of the "host environment" within which JavaScript is embedded. Usually that host environment si a web browser(though we'll see two uses of JavaScript without a web browser in Chapter 12). Part I of this book covers the language itself and minimal built-in API. Part II explains how JavaScript is used in web browsers and covers the sprawling browser-based APIs loosely known as "client-side JavaScript".
Part III is the reference section for the core API. You can read about the JavaScript array manipulation API by looking up "Array" in this part of the book, for example. Part IV is the reference section for client-side JavaScript. You might look up "Canvas" in this part of the book to read about the graphics API defined by the HTML5 <canvas> element, for example.
This book covers low-level fundamentals first, and then builds on those more advanced and higher-level abstractions. The chapters are intended to be read more or less in order. But learning a new programming language is never a linear process, and describing a language is not linear either: each language feature is related to other features and this book is full of cross-references-somtimes backward and sometimes forward to material you have not yet read. This chapter makes a quick first pass through the core language and the client-side API, introducing key features that will make it easier to understand the in-depth treatment in the chapters that follow.