Port forwarding has three (or four) parameters:
- external port number
- internal IP address
- internal port number
- optionally, whether to forward TCP, UDP, or both
Assume you are hosting a web server, then you put port 80 in both external and internal fields.
But if you want to host this server for your friends only, you can set external port to something non-standard, like 12345, but still interlan port to 80.