Port forward is telling your router which ports it should open to the outside world (connections outside of your local network) and where to direct traffic that is directed to those ports.
Some software requires certain ports to be open on your router so that they can function properly. Some Peer-2-Peer software requires ports to be open for communication to occur between online servers, etc. If you plan to stream media from your home PC to your iPhone or mobile device, odds are you will need to open a port for that communication to happen.
Port forward example:
When you have a device connected to your router/network, it will have a local IP address. For example 192.168.1.10. Let’s say you have two computers connected to your network, a laptop, and a desktop. The desktop holds a lot of your media files that you want to stream to your iPhone app with a server/iPhone app called ServeToMe and StreamToMe. That media server running on your computer will want to communicate over a port, let’s say port 5584.
What you will want to do is FIRST set your desktop machine to have a STATIC IP address, so it’s ALWAYS the same. Let’s give it the address of 192.168.1.150. Then we will log into our router and go to the port forward section settings section. Add a new forward called MEDIA SERVER, set the port value of 5584. If they give you a from and to the field, put 5584 for both. Then select the protocol, it will either be UDP or TCP, or both. You can just select both for this example. Then the last thing it will ask is the target IP address. Enter your desktop IP address of 192.168.1.150. Save the settings and you are done!
Now your iPhone app can communicate with your 3G network through the internet to your router and then through to your PC. That’s what port forwarding does. If you didn’t set up this port forward, the iPhone app would get to your router and then your router would block the traffic because there was no rule setup.
One thing to note regarding the app and the address you give to it. You will NOT give it the 192.168.1.150 IP address, as this is your local IP address and only within your local network. You need to give the mobile app your GLOBAL IP address which is IP address of your router. I have a good explanation on how to setup your global IP address with a domain name service.