What is a Registration Authority?

"The RA ensures that the public key is bound to the individual to which it is assigned in a way that ensures non-repudiation"

Registration Authority is additional element of PKI which brings control and management of users, requests and certificates. The task of Certificate Authority is to issue/revoke certificates. That's what it does. However, current complex company and business setup require more advanced functions of PKI. Those functions include user management, request management, centralization of certificate revocation lists, reports of activities, logging of certain events. Such advanced functions allow to create more complex systems and built inside business architecture. These advanced functions are under control of Registration Authorities.

In summary, Registration Authorities adds additional level of control and management to create a controlled and secure environment.