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. It allows programs to publish the existence of services and the IP, port and machine name running those services. It does not provide any actual communication -- it simply allows for service discovery, sort of like a phone book for programs on the local network. Individual applications then need to know how to connect to those services themselves. Because of this, it only affects network security insomuch as it reduces so-called "security through obscurity." It also does not cross routers or subnets without some involved configuration, meaning that your LAN will not be exposed over the internet. If you need to cross routers and subnets, you should be able to find the information you need via .
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.