A node is a combination of manufacturing capabilities and design components that are manufacturable in that process. They're typically named after an arbitrary dimension of a silicon feature, for example 14nm or 10nm. Your higher level design options are dictated by what you can produce at that "node" (with those masking methods/transistor pitches/sizes/electrical and thermal properties).

Would a pixel be a good analogy? It's the smallest thing you can make on your chip and that defines all the rest of your design.

Only in the same way that even pixels of the same physical size can have other vastly different properties. And that makes ranking them purely on their size totally misguided. So I'm not convinced that really helps.

It's closer to the quality of a display then just how small your pixels are. It determines how large your display is before you get too many dead-pixels (yield in fabrication). What range of colors your pixels can produce (electric properties? resistance, leakage, etc.). Whether you can blast all pixels at full brightness, or only a few (thermal properties). And indeed, resolution of the display (size of transistors).

What is missing from this analogy is the degree of layering / 3d structures that is possible. You might couple that to RGB v RGBY but I'm not really sure.