Actually in not so distant future we might be releasing a GPL version of Repeater with some enhanced features 
Anyway Repeater is a proxy component that allows to hide VNC servers with private IP address and still have them accessible from outside network. A typical configuration would look like
VNC Client (1..n) | INTERNET | COMPANY FIREWAL | Repeater | VNC Servers (1..n)
Connections to Repater are identified by a connection ID, so for a VNC client to connect to a VNC server, he doesn't have to know server's IP address, he only have to know the connection ID. I think the more detailed information can be found on UltraVNC web site.