Each cell in the board has some heat (red - highest, blue - lowest). Heat is transfered between adjacent cells (top, right, left, bottom). How fast heat is transfered is controlled by "Base thermal conductivity". There's a number of pellets that move acording to the heat flow inside the cell they are contained. The pellets will also add or remove heat to the cell they are currently at. "Mass of each pellet" controls how fast they move relative to the heat flow.