What it demonstrates, **Jonathan Chance**, is that no one has a good understanding of just what spacetime is at present. Physicists and philosophers have debated for centuries the question of whether there exists an absolute (if dynamic) canvas or if space and time can only be defined in a relational sense. The overwhelmingly predominant interpretation of General Relativity ties gravity to distortions of spatiotemporal geometry; implicit in that statement is the notion that there must be something there to carry those distortions. The mathematics of GR are easiest to use in this formulation, however it would be just as valid to construct it in a completely relational way.

Bottom line: we cannot answer the question of what it means for spacetime to be expanding until we understand what spacetime is. This is one of the fundamental questions that a successful theory of quantum gravity will have to answer.

Edit: I should add, though, in the context of this example about Universe expansion, it is important to remember that we do already know that spacetime is part of the Universe. Space and time did not exist before the Big Bang, the Big Bang created them along with everything else. Once you abandon the notion of spacetime as some sacrosanct background ruler, it becomes easier to accept that it might do something like expand.