It is similar to flattening the bottom of the sky sphere mesh to the ground.
Even with just an environment map, the model can look like it is standing on the ground, rather than floating in the air.
But this shader is a little more advanced, and it does this without modifying the mesh.
Is this material specific to a ground? Can this material work with a wall or ceiling?
its projecting to an icosphere here, but i think it should work for a inverted cube too.
Try zooming out and you will see what itās doing. 
The skybox is an icosphere, but the ground is a flat mesh right? If we are going to name this material with the word āgroundā in it, is that too specific? Does it have to be a ground or can it be other things?
no the ground is virtual.
the shader finds where the y=0 is and projects the ground there.
i think that is why its is called Ground Projected.
I can try a box to see what the outcome is.
i also want to find out what happens if a mesh goes below 0.
does it get hiddenā¦
Another thing that would be cool is the ability for the ground to receive shadows. Can it just be turned on easily? I will try and test it out. I also want to know whether the skybox image can also have an emissive map that brightens it ( as it should be unlit )?
Maybe it could be a mode within the background material ??? if it only impacts projection ?
Just let me know when itās ready so I can just soak-up the result of your efforts 
@qq2315137135 Hereās one like for you since you asked for it 
You can take it for your solution or just for the anime gif 
Your choice 
We might be able to add it as an option to the background material.
But at the moment I donāt have any time over to look at it.
So if anyone else feels they want to pick up where I left off feel free.
Or else I will have some time in a couple of weeks. 