The LDAPAuthenticator is implemented using JNDI, and authentication requests will be made by Cassandra to the LDAP server using the username and password provided by the client. At this time only plain text authentication is supported.
If you configure a service LDAP user in the ldap.properties file, on startup Cassandra will authenticate the service user and create a corresponding role in the system_auth.roles table. This service user will then be used for future authentication requests received from clients. Alternatively (not recommended), if you have anonymous access enabled for your LDAP server, the authenticator allows authentication without a service user configured. The service user will be configured as a superuser role in Cassandra, and you will need to log in as the service user to define permissions for other users once they have authenticated.
The authenticator itself is hosted on GitHub, so you can check out its repo too.