The browser is the operating system of the modern era and the browser's inspector is the tool that gives developers visibility into its working.
Firefox's Firebug was the first seriously powerful web developer tool. It and its clones for other browsers all continue to evolve rapidly, almost daily, as improvements are found for instrumenting and visualizing all aspects of the browser's operation.
Federated wiki developers tend to use Chrome so that is what we will assume in choosing terminology.
# Open
Use Option-⌘I to open and close the inspector. It appears within or beside the browser's window and can be moved about without interruption. Opening the inspector changes some browser behavior.
The browser collects more data while inspecting.
The browser stops on breakpoints while inspecting.
The browser can bypass caches while inspecting.
Use ⌘R to reload a web page with the inspector open. Reloading brings the loading process into view which is important. It also restores synchronization with servers if this has been lost and loads freshly built code when present.
# Learn
The browser wars continue with vendors struggling to attract the best developers with ever improving tools. Automatic updates from google have the inspector changing dramatically in look and feel from week to week.
A search for "chrome inspector" shows that this name is now obsolete. Still, it found useful documentation that you should read. chrome 