The commands in this article can be used on any Ubuntu/Debian machine.
A transparent bridging firewall is a firewall which can be inserted anywhere on a network, but usually between the network segment containing internet access and the rest of a LAN. Generally they are used to silently police and log traffic from the network to the internet and vice versa, the main advantage being that they can easily be inserted and removed without any network reconfiguration.
Further to this the segment between and including the bridged firewall and internet router can be considered a DMZ where internet facing servers can be placed. Personally I think it is a good idea to place all servers in this no mans land as they are as likely to come under attack from Windows clients on their own LAN as any hacker from the internet. The bridged firewall provides protection for both sides.