This is the Cassiopaean term for a division of reality. The use of the term is very broad. This includes the following meanings:
Density - the degree of development of a consciousness determines the general type of experience of space and time, manner of perceiving other entities etc.
STO and STS divisions of a density - Souls of different polarities may occupy distinct realms within one density.
Lateral divisions within a density, as in parallel universes.
Realms are bounded by a so-called frequency resonance envelope. A realm curtain or realm border as another word for a section of such an envelope.
The Wave is also called a realm border.
On one hand, realms are naturally occurring divisions of reality, on the other hand realms are influenced by their occupants. There is a sort of resonance effect which tends to lock the resident of a realm within its boundaries in terms of frequency. A realm might be compared to a radio broadcast and the being to a receiver. Nature provides a sort of automatic fine-tuning which will cause the receiving frequency of the being to snap to match that of the closest realm. There may be a small reciprocal effect from the being on the realm but this would be negligible in the individual case.
A realm border could be likened to many stations broadcasting near the same frequency, so that a small change of individual tuning may cause the tuner to snap to an entirely different reality. We could say that the receiver's mental map of reality decides which broadcast can be followed and thus mentally synchronized with. The realm border is just a circumstance which brings multiple 'bands' of reality within close reach or in overlap with each other.
Passage between realms is in principle possible anytime, in some cases with technology, in other cases by attaining a somehow total purity of thought and being, aligning one to another reality. However, such a process is problematic if one has no notion of what is possible or where one may be headed, not to mention the near invincible inertia of the human condition.
A realm is neither a completely physical nor a completely consciousness based concept. We could say that a realm is a set of partitions, a bit like a sandbox where a particular type of consciousness may experience interaction with reality. These move with respect to each other according to some cyclicity and the consciousness itself moves between realms for example through reincarnation. The incarnate and disincarnate states occupy different but joined realms.
Phenomena taking place at the border between realms can according to the Cassiopaeans be technologically manipulated, which is the manner in which 4D STS beings keep a sort of net of control over the Earth and enforced changes on humanity in the context of the mythical Fall.
We cannot describe these processes in scientifically precise terms, hence we have to use words like frequency and resonance in a somewhat allegoric meaning.