IPFire makes a great effort to abstract away the complexity of a router and firewall system, however there is no shortcut to setup a secure system. If the administrator is unaware of what happens under the hood, he/she becomes the weakest link of the system. This section points to videos and tutorials for the layman with the curiosity and/or desire to setup a firewall machine but lacking the knowledge to understand even the basic terminology. The following material requires time and dedication but does not need any previous knowledge unless specified otherwise.
This is a video series from Ben Heater that guides the learner through the network technology underlying Internet. Any machine that carries out a complex task requires to successfully solve many complicated problems. It turns out that the best way to deal with complexity is to divide it in simpler tasks stratified in independent layers. The problem of having computers communicating in a global, worldwide network was solved exactly by stratifying the tasks in well defined, independent layers each operating above the previous one. In this video series, Ben Eater will walk the learner trough each level of the stack, teaching these concepts in 13 relatively short videos presenting a well balance overview of the system, with enough details to understand how the entire edifice is built.
This video is part of the above linked series, but it is self contained and clarify how the Domain Name System (DNS) and the Border Gateway Protocol (BGP) operates.
The following link goes to a tutorial that covers the most important internet communication protocols and, while self contained, it is a nice complement to Ben Eater video series.
The following book (PDF and text) is free to download. It is more than 20 years old and some of its content is outdated. However the basics remain relevant and it can still be quite useful for learning specific more general topics (i.e. what is a netmask, how an IP address works, what is an IP masquerade and a NAT, and so on).
This is not for learning, but just a quick cheatsheet if you are trying to read some of the logs shown in the web user interface of IPFire