The fact that Plato is pointing at the sky represents his theory of the two worlds or realms. He's pointing to the world of ideas (I don’t know how it's called in English), the world where the truth and all the knowledge comes from.
Aristotle points to the floor because he opposes to that theory and he says that there is only one world, this one.
It's the Platonic idealism vs. Aristotelian realism, teacher and pupil debating
