Local IP - IP address of computer you want to direct the traffic to.
Local Start Port/End port: Port (or port range) that the local computer is listening to.
in most cases you have one port, so you would put the same port in both the start and end
External IP - allows you to limit connection to a single IP. Leave this blank unless you specifically want this.
External Start/End port - allows you to sepecify a different port number then what the program is looking for to combat same port numbers or that the ISP blocks ports like http-80, smtp-25, ftp-21, etc.
Protocal - TCP or UDP or both, this depends on what you are trying to setup.
Descritpion - what your descrition is