The fire model simulates how fire spreads through a forest. It starts with a wave of trees simultaneously catching fire, and with every tick of the simulation, a tree with at least one neighboring tree (to the north, south, east, or west) on fire will also catch fire.
The PERCENT_FULL parameter determines how crowded the forest is. Set to 1.0 (100% full), every cell in the grid will be occupied by a tree, and the fire will deterministically sweep across the entire forest. However, around 0.59 (59% full), there is a threshold, and value below 0.59 will rarely result in the entire forest being burned, while above 0.59 the entire forest will almost always catch fire.
Read more at https://en.wikipedia.org/wiki/Forest-fire_model