Create an instance of the Flask class. The first argument is the name of the application’s module or package. If you are using a single module, use name.
Use the route() decorator to tell Flask what URL should trigger the function. The function is given a URL, and it returns the output that should be shown in the user's browser.
Define a function, in this case, hello_world(), which will be called when the specified URL is requested.
Use the run() function to run the application on the local development server. Set debug=True to enable debug mode, allowing the server to reload itself on code changes and provide a helpful debugger.