How this works, in short, is that RWGMixer consists of multiple "modules" that are linked together. Some modules generate signals (like the Voronoi and FastTurbulence modules in your example), others take signals as input from one or more other modules and combine or change them (like scaleOutput and biasOutput).
If you want to really know how these work and interact together then check out the link below. It has helped me a lot to get a grip on it all:
http://libnoise.sourceforge.net/docs/index.html
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A little warning though. This rabbit hole can go pretty deep (just ask Tin, he lives in one

). Before you know it you'll be pulling all nighters writing stuff like this: