I’ve completed my explanation of Bison and posted it here. Bison is a context-free parser generator. My page has a brief description (considering there are books on the software) of how to form a Bison file. I go through the bison file (grammar.y) used in the PlayPen skeleton project provided in Chapter 0 with comments to explain each grammar rule. An understanding of Bison is important at this particular point and time because this is where and when you determine the syntax of the language (or your language if you are deviating from the tutorial, which I encourage). The computer language must be able to be parsed by the rules set in this file.
I should mention that I used Win Bison because I wanted to be able to compile and run everything on Windows for the tutorial. However, while preparing this tutorial, I used GNU Bison, so everything should work for those using Linux.