A grammar is defined that handles strings of symbols.
Axioms define the initial string of symbols. Here F -- F -- F will be the starting point.
Rules allow some symbols to be rewritten ( through substitution). Thus F may be rewritten as F + F -- F + F.
Some symbols have a predefined meaning to be used in the final stage.
A number of directions defines an angle value. Here 306°/6 = 60°.