A Pillar of Creation. They're worlds where the energy of Creation flows through more strongly, so they hold up more of reality. [ She pauses, frowning as she tries to think through what she knows and explain it in non-crazy terms, which is nearly impossible. ]

Basically, there are a lot of universes where 'Earth' exists, right? But an Earth that's a Pillar of Creation, like Wismuth -- it's a little more 'real' than the others. It's not just a place ... its existence is tied to the concept of the Earth existing itself. Destroying it would affect all Earths, everywhere. If enough of them get destroyed, 'Earth' would stop existing completely. In every universe.