Widely used thought device
when trying to free the mind from the constraints of our 3D thinking or something like that.
It is basically a two-dimensional universe, with 2D beings, who can't perceive the third dimension.
You can then suggest allsorts of experiments to prove that this third dimension does in fact exist.
Yep it's just like living as a pixel in the middle of this computer screen :)
Onwards, to more crazy mathematics