Chrome Developer Tools, aka DevTools, are a useful set of Web development and debugging tools built into Chrome. They include panels for Elements, Network, Sources, Timeline, Profiles, Resources, Audits, and Console.
You can open DevTools by selecting the Chrome hamburger menu at the top right of your browser window, then selecting More tools > Developer Tools. Alternatively, you can use Ctrl+Shift+I (or Cmd+Opt+I on Mac). Once the DevTools are open, you can use Ctrl+[ and Ctrl+] to move from panel to panel.
One of the most common uses of DevTools is to examine the HTML and CSS for an element of the Web page currently displayed in Chrome. This is achieved by clicking the Elements tab in the DevTools navigation bar. You can move the focus to a different element of the Web page by right-clicking on a page element in the Web page view and picking Inspect Element. You can also live-edit your CSS in the Styles pane on the right. This is a great way to see how to alter the look of a page in progress without having to make the round-trip to your code editor and back.