fwbuilder is not part of the OpenWrt package tree
It does not appear to have been updated since 2013. This article is retained as reference; fwbuilder appears to be good way to learn netfilter rules.
Fwbuilder is Free Software allowing to generate complex firewalling rules for iptables, including detailed logging and much more. A detailed feature list of fwBuilder can be found on fwBuilder home page.
fwBuilder GUI is available under the GPL licence under all platforms: GNU/Linux, FreeBSD, Mac OS X and Windows.
Under Debian GNU/Linux, install fwBuilder using these commands:
sudo apt-get install fwbuilder
Do no work on a production firewall while learning fwBuilder.