If you have other machines attached to an ethernet (for example a home LAN, other machines in the office, or your cable modem), Lokkit will notice them. For each ethernet interface it finds, you will be offered the chance to 'trust' this network. This means the firewall rules will not be applied when machines on this network are talking to your Linux server. However, your Linux server will apply the same protection to machines on this ethernet as it does to itself.
If the other machines are behind the firewall and do not talk directly to the internet then you can select Yes.
If the ethernet is for your cable modem, however, you should select No.