Yet another example:
I have a webcam behind a firewall/router. I tell my webcam server software to request data from the IP address of my router (as my pc with the webcam has an ip not on the internet, its local to my lan only) I also tell the software to request from port xyz. Now I tell my router to "forward" all requests on port xyz to my the local ip address of the pc with the webcam.
Seems simple now, but boy was it a pain when I first did it.