This model represents the solution to the diffusion equation with the initial conditions of the Heavside function, H(x).

You can think of it like this: Imagine you have a thin metal bar and heat up one side, so that it is uniformly at some temperature, and the other side is uniformly at a cooler temperature. As time passes, the heat will flow from the warm side to the cool side. The position on the bar and time are the two axis, while heat is the height.

Since the diffusion equation also governs things like the diffusion of a chemical in water, there are many other possible interpretations. Read more at Wikipedia:

secure.wikimedia.org/wikipedia/en/wiki/Diffusion_equation

This model was made with one of the predecessors to surfcad. You can read more here:

christopherolah.wordpress.com/2011/07/16/surface-oriented-cad-math-telescopes/