Authentication is the process of verifying the identity of a user, system, or entity. In a AAA (Authentication, Authorization, and Accounting) framework, authentication establishes the identity of the user before access to resources is granted. It is the first step in the AAA framework and is crucial for ensuring that access to systems and data is granted to the right entities. Authorization, on the other hand, determines what an authenticated user is allowed to do, and Accounting tracks what the user does.