As the title implies, this is the "Advanced" HOWTO. While by no means rocket science, some prior knowledge is assumed.
Here are some other references which might help teach you more:
Very nice introduction, explaining what a network is, and how it is connected to other networks.
Great stuff, although very verbose. It teaches you a lot of stuff that's already configured if you are able to connect to the Internet. Should be located in /usr/doc/HOWTO/NET3-4-HOWTO.txt but can be also be found online.