Rabbit_L system_turtle

Hi

As i have just discover it two days ago while looking for some branching tutorial on grasshopper forum the morphocode website.
By curiosity, for learning purpose
While learning Rabbit i am wondering:
How does from the input of L system: process Strings (words), letters and rules, output words to Turtle input then resulting from turtle output
A geometry ? Is their other than logic in this two components? Cause geometry in my knowledge cant be simply produced by rules ?

Thank you again forum www.grasshopper3d.com