The application allows users to shorten URLs, with features such as storing original and shortened URLs in a database, checking for existing shortened URLs, and displaying errors if the input URL is invalid or already shortened. The application uses PHP's Slim framework, Twig templating engine, and PostgreSQL database to manage its functionality. It includes two routes: one for shortening URLs and another for retrieving un-shortened URLs. The application also features a 404 error page that redirects users back to the main route if they attempt to access an invalid shortened URL.