The Realms refers to the multiverse of distinct worlds and universes that intersect through the In Between. Different realms connect along set pathways. Most of the time, the pathway is blocked, necessitating the use of Riftmancy to break the blockade and travel to another Realm.
The Seraphim fancy themselves the police of the Realms, and enforce strict travel restrictions between realms, often hunting riftmancers ruthlessly to prevent contagion. Earth has become an intersecting world between many realms, causing a crisis in the Realms. The Seraphim have yet to decide what to do about this.