As I understand it, the Poisson Mesh node converts a point cloud (a set of disconnected points in space) into a surface for rendering. How might this relate to Terragen? Well, in Terragen 2.5 you will be able to save a "surface position" element that records the 3D position of every pixel in the image (commonly called a "P Pass"). They are useful in all sorts of ways for compositing, as you can extract masks based on position in 3D space, add lighting to the image... all kinds of things. Nuke can turn this into a point cloud and, with Poisson Mesh convert to a polygonal surface. I haven't tried to generate point clouds or meshes from Terragen data though.
Matt