It is recursively changing the angle at random and shrinking the size. I dunno if that would count as a L-system, the wikipedia article for L-system has too much math that I don't understand
Track down a copy of "The Algorithmic Beauty of Plants". The last time I looked, it was available as a free PDF, too. I think it's a great introduction to L-systems.
I got it when it was first released and spent way too much time poring over it and generating imagery on my decrepit old computers.
Nice! That Vlab software looks very interesting. Way back when, I built some primitive tools to help me play with L-systems, but I'd never have dreamed of taking it that far.