Filemaker lives on a computer in your network.
That computer has a local IP address of 10.0.0.3 (or 168.192,0.3) or other
Your router has a local IP address of 10.0.0.1 (or 168.192,0.1) or other
It has a database that has IWP enabled. On port 80 or 591 or other
Your router (or DSL or Cable modem) has an IP address that is internet accessible (found with whatismyipaddress.com)
Some one outside your local network, tries to access it with filemaker or a web browser by typing your internet address and the port number http://18.104.22.168:591
Your router does not know what to do with this request unless you have told it to forward this port request to the computer hosting the filemaker database.