I guess, I understand it now.
The idea is based on an imaginary three dimensional spacetime. So, why three? The idea is to treat time as dependent on the three other dimensions. So time would be a real movement - in an imaginary space. This can't be seen at the place where we are, since that is the way we define ourselfs. At our spot, it's a movement in time.
Now think about the lightcone. Our past light cone is what we can see. Further away means something like nearer to the outside. In that imaginary space it would mean an angle increasing with distance. That angle makes out of our advance in time a movement in space.