What is an Interpreter

One could view an L-system as a kind of programming language. It describes in a declarative way how to produce a series of symbols. What we will do in this section is write an interpreter.

An interpreter is

is a computer program that directly executes instructions written in a programming or scripting language, without requiring them previously to have been compiled into a machine language program

In our case the instructions is the L-system, as parsed by our parser. Execution is producing the series of symbols for a certain level.