It should be asking for "internal IP" and a start/end port as well as a port for external forwarding.
Generally, forwarding has the ability to forward the external port on your modem (internet facing) with any internal port. Note by port, I mean port number not physical connection.
Your Internal IP will be the Internal IP address of the computer/device you wish to forward the external port number to. Range can simply be that port number again..
So, if you want to forward port 1000 for example, you enter 1000 in external port, enter the internal IP address of the computer you want to forward to, and the port range can be 1000 - 1000. It can also be different, if you wish, for example you can forward external port 1200 to internal port 1000. Range is useful when multiple ports need to be forwarded.
For most use cases it isn't really needed. I suggest if your application allows it to simply use uPNP as it will do this all for you, although you should perhaps research the security implications of uPNP as well.