Here is what the above code is Doing:
1. We’re importing the Flask class. An instance of this class will be our WSGI application.
2. We create an instance of this class. The first argument is the name of the application’s module or package. If you are using a single module (as in this example), you should use __name__ because depending on if it’s started as application or imported as module the name will be different (‘__main__’ versus the actual import name). This is needed so that Flask knows where to look for templates, static files, and so on. For more information have a look at the Flask documentation.
3. We are defining our first route here. The decorator @app.route() tells Flask what URL should trigger our function.
4. The function is given a name which is also used to generate URLs for that particular function, and returns the message we want to display in the user’s browser.