In any cellular automata (CA) program, the screen is divided up into a number of square cells.
The color of each cell represents its state. At the start, all cells are black (state 0).
The turtle starts in one cell. To determine what to do next, the turtle finds the color (state) of its current and neighbouring cells.
This information tells the turtle which cells to recolor (alter the state of) and which neighbouring cell to move to next.
The result is a pattern of different colored cells.