Nodes are the essential building blocks in the Advanced interface to the Poser Material Room. They are the graphical representation of mathematical function calls, that is: calculation procedures which turn parameters (inputs) to a result (output).
Like I can define my own surface definitions by plugging node driven effects into PoserSurface (and other root node) inputs, so I can define my own effects by combining nodes into constructions. Functions calling functions calling … and so on.
Of course the effect nodes themselves, as discussed in the various articles, are relevant bricks in such constructions. But to glue them together and to help me with the construction, the nodes from the Math group make the mortar between the bricks. And the nodes from the Variables group get the information from the scene, like the time and place of the spot on the surface for which PoserSurface is defining the material for. As node constructions can be huge, the Compound node was introduced (Poser 10/Pro 2014) to keep clear boards in the Material Room.