Authentication is the process of checking that someone is who they claim to be. It is like showing a key or identity card before entering a private room.
When you sign in, a service may check your password or another sign-in method to verify your account. If the check succeeds, the service can recognize you during that visit.
Authentication answers “Who are you?” It is different from authorization, which decides what an identified person is allowed to do. Strong sign-in methods help protect accounts from someone pretending to be their owner.