https://www.netfilter.org/ — The home of the netfilter/iptables project. Contains assorted information about iptables, including a FAQ addressing specific problems and various helpful guides by Rusty Russell, the Linux IP firewall maintainer. The HOWTO documents on the site cover subjects such as basic networking concepts, kernel packet filtering, and NAT configurations.

https://www.linuxnewbie.org/nhf/Security/IPtables_Basics.html — An introduction to the way packets move through the Linux kernel, plus an introduction to constructing basic iptables commands.