It's a bit like this.
Imagine something which is spherical. Now imagine it is also shaped like a cube.
So it is cubic and spherical at the same time. This appears impossible since these have different geometrical properties but you got to imagine this thing is both spherical and cubic at the same time because I say so.
Also it is not changing between spherical or cubic and it is not like it is a sphere painted to look like a cube or something like that; no cheating.
Now you have the best minds spend 1000 years hammering out the properties of this thing. The only thing you are not allowed to do is ask why one should suppose it exists or why it can't just be a sphere or a cube or a sphere next to a cube, otherwise you will be burned at the stake.
The end-result will be a bit like the trinity.