The simplest application might be fully contained site with just a URL as part of its configuration — like site that has a single service that serves static pages.
A more normal application might be a website like a blog configured with a URL and a database, with a single service that serves HTTP requests.
A complex application would be a shopping site that had multiple databases and caches like a MySQL database and Redis cache — which are accessed by one service configured with a URL, and also by many background tasks that collect inventory information, co-ordinate with shipping services, etc.
If you're using more than one environment, like a separate testing and production environment, the Application construct helps you make sure that the same configuration keys are applied on both environments. The values of these keys might be different in each environment, but CONTROL will show you diff to make sure you haven't missed anything out.