The password clients of the admin interface have to specify in order to be able to use it. A good password should be chosen for security reasons, as the admin interface can be used to control the server (it supports sending RCON commands without any extra check for the RCON password). If no password is set the admin interface isn't activated and no listener is started on the port specified in Server admin port. Even if the listener was started the server would reject any request for that interface as if the password was wrong, making sure the admin interface cannot be used with an empty password.
More information on the admin interface can be found here: http://svn.openttd.org/trunk/docs/admin_network.txt
Valid options: [Empty text , Password text]
Default: <Empty> (= admin interface disabled)
Example with password set:
[network] admin_password = MySecretAdminPass
Example without password set (admin interface disabled):
[network] admin_password =<<Back to the Openttd.cfg