Android : libgdx, how to create a object into 3d world

I trying to study the libgdx framework to develop a simple 3d game in android devices. I search and read a lot of stuff, but I don't really understand how 3d objects are building. I tried to use the model and modelbuilder classes to create a simple box or sphere, but i don't know to use other classes 'cause I cannot understand the link between shader, texture, mesh, material and attributes. Is there anyone that can explain me how all of this work ?

thanks