You mean the UV layout?
Basically the modeler transfers the 3D surface to 2D, while using maximum texture space. Each polygon has its own space on the texture sheet so it can be painted on.
The scientific version:
"UV mapping is a 3D modelling process of making a 2D map representing a 3D model. This map is associated with an image known as a texture. In contrast to "X", "Y" and "Z", which are the coordinates for the original 3D object in the modelling space, "U" and "V" are the coordinates of this transformed map of the surface. Then the image is back-transformed ("wrapped") onto the surface of the 3D object."