From the 'Game Design Tutorial' on the Download page:
For each triangle (also called face or polygon) in the mesh, a corresponding triangle is cut from the model texture image (alse called skin). Or, in other words, for each three-dimensional vertex of a triangle that describes a XYZ position in space, there is a corresponding two-dimensional vertex that describes a UV position on the skin picture. This method is called UV mapping. It is not necessary that the triangle in 3D space and the triangle on the skin have the same shape (in fact, it is normally not possible for all triangles), but they should have shapes roughly similar, to limit distortion and aliasing.
With MED there are three different ways to bring a skin onto a model: Atlas mapping, two sided mapping, and six sided mapping. Atlas mapping is only supported by very new MED versions, so here I'll concentrate on the two older mapping methods. By the way, what the heck is a mapping at all? It is just the distribution of the UV vertices on the skin image, also called skinning. I normally use the six sided mapping because it offers more advantages than the other method.
When you download it, the file is called 'wed_tutorial_e.zip', but it is about MED as well.