ok go to the website i posted...
$IPTABLES and $INSIDE are "shortcuts" i guess is a good enough way of putting it.
At the top of the sh script I have written to do my firewall I have set DEFINITIONS
$IPTABLES and $INSIDE are two of them.
it would look something like this:
saving me typing this out 100 million times.
$INSIDE in this case is relating to the INSIDE ethernet card
you can also use it to help creating your scripts easy so in the example of port forwarding the computer you are wanting to forward to may change for some reason. So instead of changing EVERY rule that affects that PC you will set a definition
then rules will look like
$IPTABLES -a INPUT -i $OUTSIDE -d $WEBSERVER blah blah
Hope this helps.. but if you want to find out more check out that site or look up iptables tutorials or how tos