The application allows users to sign up for a newsletter by filling out a form with their name and email address, confirming their subscription via an email link sent by the system, and then unsubscribing from the newsletter. The system also enables authors to upload a newsletter HTML file and send it to all subscribers at once. The application uses ASP.NET Core, Entity Framework Core, and SendGrid for sending emails. The system can be improved with additional features such as authentication and authorization, using a more efficient database, and adding scheduling functionality to the form on the Upload page.