A .mtl is a material settings file used by 3D object editing applications; stored with an .OBJ files and describes how textures are applied to the object; includes the name of texture bitmap files as well as the 3D positioning of the textures.

Basically the mtl file tells the application how to apply the textures to the object it coresponds to. You still need the textures. IF you have the obj you can use a app called UV mapper to create a base texture file and then create your own textures from there.