Port Forwarding is a special function on the router, allowing transfer of data packets from the outside (from the Internet) to devices or computers on your local network (LAN). Usually the port on your router will be closed to prevent hackers on your system. But when you use the service to connect through port on the router you need to open it. For example: Yahoo! Messenger you need one of the following ports must be open: 5061, 443, 80.