The Source SDK is the development kit for creating content in Valve's game engine, the Source engine. In The Beginner's Guide, it was used by Coda to make his games, which act as the different levels the player visits while Davey comments and modifies them on-the-fly.
It cannot be overstated how important the notions of modding and creating maps on this engine is to the full understanding of the concepts behind the game - and not only for the basic understanding of the story. This article goes into it from the viewpoint of a renowned Source Engine modder in the exact time-frame explored by the game.
In Down, Davey goes as far as to mention how understanding the engine is crucial to understanding the narrative being presented. Ironically though, in what is hardly a coincidence, as soon as he mentions that the engine lends itself to creating "boxy, linear corridors", the basement of a café turns into an abstract space without walls, featuring a huge, bizarre drop with abstract shapes spread throughout.