Kerberos is a protocol that allows for secure authentication even over unsecure networks. It can be difficult to set up, but after it is fully working you have a very secure authentication system with the benefits of single sign-on.

It is usually a good idea to first create a working setup where the server uses username and password/LDAP authentication and a user directory in LDAP mode, and then proceed with switching from username and password/LDAP to Kerberos.