The Java Authentication and Authorization Service (JAAS) is a Java-based framework that provides a way to secure applications by managing user authentication and authorization. It allows developers to implement security features by defining how users are authenticated and what resources they can access within an application. JAAS separates the authentication process from the application logic, enabling a flexible security model. It supports various authentication methods, such as username/password and token-based systems, and can be integrated with other security frameworks like Java EE and Spring Security to enhance application security.