This article is about building extensions for Google Chrome Developer Tools, also known as Chrome DevTools. Specifically, you'll learn how to work with element that is currently selected in the 'Elements' panel. Prior knowledge about DevTools extensions is required. Check out Boris Smus's Extending Chrome Developer Tools for an introduction and read through Google's own documentation on Chrome Extensions.
You can build extensions for the Google Chrome browser itself, which are targeted at regular users, as well as extensions for Chrome DevTools - the set of panels that developers use to inspect and audit content on web pages.