Name server settings should not be altered unless you are absolutely sure that the name servers should be changed so the domain can be used with another host.
External domains are domains that are registered at another company and currently point to 1&1 name servers (DNS). If your domain is listed as an external domain in the 1&1 Control Panel, you will have to update which name servers should be used with that domain at the domain's registrar, not in the 1&1 Control Panel.
A confirmation page is displayed. It may take up to 48 hours until the new name servers set are recognized by all servers/computers on the Internet.