Arrow showed that when a group chooses, there are no underlying preferences to uncover--not even in theory. In one sense, the theorem is trivial. We know or should always have known that a group doesn't have preferences anymore than a group smiles. What Arrow showed, however, is that without invoking special cases we can't even rationalize group choices as if leviathan had preferences. Put differently, the only leviathan that rationalizes group choice has the preferences of a madman.
the whole glorious thing here