This is an implementation the forest-fire model. The green cells are trees, the black cells are empty spaces, and the red cells are trees that are on fire. Fires are started through lightning. When a tree is on fire, it will die and spread the fire to trees in its Moore neighborhood in the next step.
For a given step, Pgrowth is the probability of a tree growing in an empty space and Plightning is the probability of lightning striking a tree.