In metaphysics, this concept relates to how the properties of our being are able to communicate.
The 3 concepts of being: "I", "I Am", and "I Am That".
The known - relates to "I Am That": These are the attributes of "I" which we assign to what we consider the real world. What we can see, touch, hear, smell, taste, etc...
The unknown - relates to "I Am": This relates to non-factual attributes of our lives. What i mean is that something can be imagined or pictured. We can relate to it (like power animals or spirit guides), but they are outside of our ability to prove. For instance, not knowing about a physical object or skill, like how to swim or ride a bicycle still applies to the known. But Dragons and Unicorns would apply to the unknown.
The unknowable - relates to "I": This is the most distant attribute within our existence and can be damaging to our psyche if we pursue it too determinedly. But we are not "never" in touch with it. As we evolve, the unknown will pull in fragments of the unknowable and therefore becomes safe to pursue.
All of these ideas can be difficult to explain clearly, so let's leave the outlines above and work with descriptions and allegory.
In metaphysics, a boundary is like a shield between what we believe to be the real world and everything else. Beyond it might be parallel dimensions, unknown beings or those of myth, also information that may provide the how and why of our existence. There are boundaries between the known and unknown, and the unknown and unknowable. They are there for a very good reason because they allow us to live somewhat normal lives.
We manipulate the boundary between the known and the unknown to a good degree, depending on the type of life we lead. For instance, Artists, Musicians, Authors and others within creative careers or hobbies have a more permeable boundary. A significant portion of creative inspiration comes from the unknown.
The unknowable is an attribute of the universe beyond our comprehension. It is not something our brains have the capacity to endure. Nor can we manipulate the boundary between the unknown and the unknowable. But to those in the unknown space to us... well to those, our unknowable is their unknown.
Tough stuff to mentally organize, isn't it?
Some of the things that separate us are there to protect us. But what is more bizarre is that this boundary is also a paradox.
It also controls the speed of our evolution. The stronger the veil, the slower we evolve. This boundary does not only exist for the individual. It is also a universal ideal similar to the way that music (IE... organized vibration) is considered a universal language. The Earth has it, our Solar System, the sun, our Universe, and so on.
Those who seek, explore and create are the ones always pushing the boundary. Thus they will evolve more quickly. The flip side are people like politicians, executives and the like. I can't speak for every one of course, but for the most part they deal in real world concepts. So much so that they effectively close off these other portions of their brain. They encase their barriers in iron so nothing gets through.
Yes, we build these barriers ourselves.
To evolve, we need to establish some kind of balance. We want to break down this barrier but at a speed that allows us to adapt and integrate it into our lives. Too little a barrier, and we are lost in a new-age type world. We can't stay grounded and the mind flits around recklessly, unable to keep focus upon the job at hand. Too solid a barrier, and we narrow our focus to the point at which the job at hand is all that we know.
Geographically, we have the mindset of too solid a border. We build walls for everything. Continents, countries, states, counties, towns, yards. Some are necessary. Some narrow our vision. If we got rid of all borders beyond towns and yards, all governments of the world would fall apart. They would no longer have any point of reference to govern. See how dependent we are upon borders?
Our evolution requires an understanding of these boundaries. The functionality is the same as the way we live our lives. Not too much. Not too little. We should grow, learn, create and dissolve our borders in such a way so we can adapt and evolve.