very simple, for the sake of simplification:
- each computer in your network is identified by its own IP (assigned by router)
- each computer has "ports" that lets outside connections interact with your computer for specific purposes, these ports are numbered
- your router functions as a "gate" and blocks all special connections unless you tell the router specifically to let certain things through
- outsiders only know your external IP (the one assigned by ISP), and the port that they want to use to connect to your computers
- so you tell the router to forward the connection to different computers when different ports are requested
here is a sample network:
modem -> router -> computer 1 (192.168.0.2)
-> computer 2 (192.168.0.3)
if you want to run a ftp server on computer 2, first you need to set up the right software on it, then set it to "listen" to port 21 for incoming connections (like waiting for someone to knock your door).
then you tell your friends your external IP and the port number you are using, which is 21 for most standard ftp servers
however the router is still blocking this, therefore no one can get through the gate to knock the door
so you tell the router "if a connection is requesting to connect to port 21, then forward it to 192.168.0.3"
in your screenshot, here is how you fill in:
"name" - can be anything you want
"public port" / "private port" - usually i use the same number.
"IP address" - the internal IP of the target computer
