I think GP is referring to the heat equation. "species" is the concentration of the "stuff" that you're describing mathematically. Call that u(x), where is a spatial coordinate and u is a real valued function.
Then the diffusive part says
du(x, t)/dt = \nabla_x u(x, t).
The \nabla term is the laplacian: a multivariate form of the second derivative.
The equation says that a short time from now, u(x, t) will change in proportion to the average value of u, calculated over a small ball surrounding the point x, minus the value of u at the point x itself.
If there's less "stuff" in the points that neighbour x than at x itself, the function will decrease over time. Similarly if there's more stuff at the neighbours of x, u(x, t) will increase. This is the basis of diffusive behaviour.
(Edit: I think the equation in the article is wrong, unless I've misunderstood something: they have a delta (first derivative) when they should have a nabla (laplacian))
Then the diffusive part says
du(x, t)/dt = \nabla_x u(x, t).
The \nabla term is the laplacian: a multivariate form of the second derivative.
The equation says that a short time from now, u(x, t) will change in proportion to the average value of u, calculated over a small ball surrounding the point x, minus the value of u at the point x itself.
If there's less "stuff" in the points that neighbour x than at x itself, the function will decrease over time. Similarly if there's more stuff at the neighbours of x, u(x, t) will increase. This is the basis of diffusive behaviour.
(Edit: I think the equation in the article is wrong, unless I've misunderstood something: they have a delta (first derivative) when they should have a nabla (laplacian))