"EVERY_N_MILLISECONDS" One of the FastLED examples that I've seen was using code that looks like this:


  EVERY_N_MILLISECONDS( 300 ) { transition_step(); }


I've copied it and successfully used it in the same context - but I have no idea what it is or how it works.

A Google search finds no reference anywhere at all. 

I'm still a beginner with C but this looks like a Define to me - all being in uppercase - is it defined somewhere in the FastLED library?
Yes, it is defined in the FastLED library.  It is a preprocessor macro which is replaced by a block of code when compiled:

It is a very convoluted bit of preprocessing code that is difficult (even for me) to follow through with lots of concatenations and multiple layers of definition, so my advice is to just not worry about it and just use it.

If you want to try and decypher it all it's in the file lib8tion.h in the FastLED library.