Those are two different facet of any users.
The opposite of Local users are Remote users, who are authenticated by a remote authenticator over LDAP, RADIUS, TACACS+, etc. Local users are authenticated by the FortiGate(FGT) itself based on the user configuration.
Administrator users are configured under System->Administrator, and who accesses the FGT and configure it and troubleshoot issues releated to the FGT. Non Administrators are regular/general users who send/receive traffic through the FGT but don't have admin access to it.