Check out wikipedia for a pretty good explanation of l-systems. Here's a quick summary: The picture on the left is generated by "drawing" the string generated by the parameters on the right. How is the string generated? How does a string get "drawn"?

How a string gets generated:

Here's how the default example gets generated:

  1. X (we start with the axiom)
  2. F[+X]F[-X]+X (All the X's were replaced with F[+X]F[-X]+X)
  3. FF[+F[+X]F[-X]+X]FF[-F[+X]F[-X]+X]+F[+X]F[-X]+X (All the F's were replcaed with FF and all th eX's were replaced with F[+X]F[-X]+X)
  4. As you can see this string grows exponentially so imma stop right here but imagine doing this 6 times

How to "draw" a string

forward distance
turn in degrees