The most basic polygon your 3D card can draw is a triangle. By tessellating these polygons in a grid like pattern a kind of mattress is drawn. Since polygons share points on the mesh, the complex mathematical operations required for each point can be shared with adjacent polys. Your 3D card will render the triangles, each polygon can be plain or more commonly textured. Draw all the poylgons in your mesh and a bumpy mattress is rendered.
More triangles mean smoother hills.
This creates a bit of dilemma for programmers and users alike. All that information on the height of each point needs to be stored, with three points to a triangle and two triangles to a square, horizon distance and triangle size determines how much data and number crunching is required each animation frame. But if you could see the edge of the world you would see the mesh wraparound - and where it joins, the texturing of the terrain stands out clearly. The mesh data in Longbow 2 does not require much storage space, the big disk eater are the ground textures which cover the whole map and need to be decompressed in large chunks. As mentioned earlier, for low level flying, more detail is needed for immersion.