The In-Between is a poorly-defined realm that seems to exist in the cracks between worlds that are already cracks between worlds. According to The Cat, a being that is apparently a native of the in-between, it is the realm between "Day and Night, Awake and Asleep, Dark and Light, Cold and Hot, Tick and Tock, Dead and Alive, Real and Imagined".
During The Meeting, the inbetween is allowed to leak out of... wherever it is and take over the city. It has properties that resemble the real world, The Dreamscape, The Hedge and even Fae Arcadia. The Cat seems to be able to summon "pockets" of it, but not without great effort.