Besides the diagram you'll find tutorials on https://www.frozentux.net/category/linux/iptables/ https://www.frozentux.net/category/linux/iptables/ too. And at

Besides the diagram you'll find tutorials on https://www.frozentux.net/category/linux/iptables/ https://www.frozentux.net/category/linux/iptables/ too. And at http://www.easyfwgen.morizot.net/ http://www.easyfwgen.morizot.net/ there's an old, but still useful generator for an iptables setup. That should help to understand iptables.