This is a Cassiopaean or Pleiadian term for an artificial construct meant to occlude perception or keep someone's perception or FRV within a particular range.
The term was first used in Bringers of the Dawn and then mentioned a few times in the Cassiopaea material. The two sources seem to use the term in a similar meaning.
We cannot give a precise definition of the term. We can think that consciousness has a frequency and that this frequency binds it to a realm where this frequency fits. This is similar to a radio tuner not receiving transmissions which are off the selected frequency even though their signal physically reaches the antenna. The frequencies to which the receiver is not tuned are invisible, as it were. A frequency fence is a structure which keeps receivers and transmitters it affects tuned to a certain band. The idea is insidious in that the frequencies that one cannot access for all intents and purposes do not exist. Thus it is difficult or impossible to prove the existence of such a thing, specially from inside it.
The effects of such a thing would greatly vary depending on the affected persons' natural tendencies. If one naturally stays within the fence one need never become aware of it. Further, distinguishing artificial limitation from natural limitation is problematical.
In practical terms, a frequency fence may manifest as any artificial control on emotion, however usually tending away from consciousness, towards indifference or depression or towards chaotic activity or obsessive preoccupation.
The only defense against such can come from awareness of the possibility of the fence and of knowing oneself. Having these, one can use will to combat the effects of the fence or to avoid situations where it is felt the most.
We note that this need not be a planet-wide situation in all cases. The effects may be localized or be transmitted through particular people. The Wave and Adventures series at the Cassiopaea site discuss many unusual effects that may be linked to a phenomenon of this sort. Saying anything conclusive is next to impossible but the idea of electromagnetically influencing the mind is well demonstrated. Thus there is corroboration for the general concept, although its hyperdimensional aspects are not fully understood.
From Bringers of the Dawn:
' It was like corporate raiding on Wall Street. These creator gods raided Earth approximately 300,000 years ago-the time period, historically speaking, that you would call the beginning of human civilization.
When this skirmish occurred, a certain group of entities fought in space and won the territory of Earth. These new owners did not want the native Earth species - the humans - to be informed of what took place. Uninformed, the species would be easier to control. This is why light is information and darkness is lack of information. These entities beat out light, and Earth became their territory. It gives you a new idea of light, does it not? There was great radioactivity and nuclear action, and much of Earth was rent asunder. The original species, human creation, experienced great destruction and was scattered. These new creator gods who were the new owners were also master geneticists. They understood how to create life, and they wanted this territory for their own reasons. Territories are created and held by certain energies for many reasons, one of which is that there is consciousness within all things. Consciousness communicates continuously. Consciousness vibrates, or can be led to vibrate, at certain electromagnetic frequencies. Electromagnetic energies of consciousness can be influenced to vibrate in a certain way to create a source of food. Just as apples can be prepared and eaten in a variety of ways, consciousness can be prepared and ingested in a variety of ways. Some entities, in the process of their own evolution, began to discover that as they created life and put consciousness into things through modulating the frequencies of forms of consciousness, they could feed themselves; they could keep themselves in charge. They began to Figure out that this is how Prime Creator nourished itself. Prime Creator sends out others to create an electromagnetic frequency of consciousness as a food source for itself. The new owners of this planet had a different appetite and different preferences than the former owners. They nourished themselves with chaos and fear. These things fed them, stimulated them, and kept them in power. These new owners who came here 300,000 years ago are the magnificent beings spoken of in your Bible, in the Babylonian and Sumerian tablets, and in texts all over the world. They came to Earth and rearranged the native human species. They rearranged your DNA in order to have you broadcast within a certain limited frequency band whose frequency could feed them and keep them in power. [ ] Anything [of DNA] that was unnecessary for survival and that could keep you informed was unplugged, leaving you with only a double helix that would lock you into controllable, operable frequencies. A frequency fence, something like an electrical fence, was put around the-planet to control how much the frequencies of humans could be modulated and changed. As the story goes, this frequency fence made it very difficult for the frequencies of light-information-to penetrate. ' [End quote}