The function of an application server is to host and manage web applications, providing the necessary environment for them to run. It acts as a middleware layer between the web server and backend databases, handling requests from users and facilitating communication between different software components. Application servers typically execute business logic, manage user sessions, and ensure data security. They often support various programming languages and technologies, allowing developers to build scalable and robust applications. By optimizing processing resources and providing essential services, application servers enhance application performance and user experience.