Java Authentication and Authorization Services (JAAS) is a set of APIs that enables user authentication and authorization within a Java application or service. It provides a framework for incorporating pluggable login modules that can verify user identities via various authentication mechanisms, such as passwords or digital certificates. JAAS also supports authorization policies that grant or deny access to specific resources or operations based on a user's roles or permissions. Overall, JAAS helps developers implement secure, flexible, and customizable authentication and authorization mechanisms for their Java applications.