The Maelstrom is an energy field that seperates parallel universes. Conceptually, each universe is a pane of glass that is kept apart from other universes by the energy of the Maelstrom. If two parallels are similar the Maelstrom will be “thin” and travel or contact between them will be “easy”. If they are wildly different then the Maelstrom will be thicker and more energetic making contact all but impossible.
Moebius is very different from other universes and is effectively cut off. But it is reachable in certain places or times which is how the Gods and certain other visitors have reached it.