Bonjour is Apple's zero-configuration discovery service. modo uses Bonjour to find other machines on the LAN for network rendering and licensing.
Bonjour uses multicast DNS (mDNS) to broadcast services to other machines on the network. mDNS does not cross routers without special configuration of the routers themselves, meaning that no Bonjour traffic will leave or enter your network, thus making Bonjour very safe if you are concerned about network security.
Bonjour uses UDP port 5353. This port needs to be open on your firewall for modo to be able to see other machines on your LAN. The Bonjour installer normally sets this up automatically.