This option enables you to specify a authenticator class to be used for user authenticator. A basic session authenticator is build in this one is called SessionAuthenticatorImpl and it checks specific session keys for access data.
If you create your own authenticator class you need to include that .php file in you config.php and add the authenticator class name to this option.
Example of usage in the config.php file:
$mcFileManagerConfig['authenticator'] = "SessionAuthenticatorImpl";
Example of usage in a mc_access file:
authenticator=SessionAuthenticatorImpl