What is the Fluorescent Zone?

The simplest way of understanding the Fluorescent Zone is to imagine it as three separate solar systems, wherein time and space do not travel linearly.

If "Zone 1" represents the reality known by you and I, it's best to think of it as the Milky Way, as it includes Earth, the sun and moon, surrounding planets, etc. that you know about from textbooks. It's the smallest section shown in the middle in the diagram above.

"Zone 2" and "Zone 3" used to be one shared world that represents the realm of fantasy and dreams, known as the Hidden World. However, a great war broke out and caused their division into separate districts.

Humans may still natively dwell in Zones 2 and 3, but they aren’t the predominant race. Animals with human-like qualities, aliens, living plants, and new creatures entirely all get along in Zones 2 and 3.

How does one travel between zones?

Anyone can travel to any planet shared within the same zone through standard space travel. But travel to the other two zones is unpredictable and usually not intentional, though some may try hard to achieve it. Certain strategies do exist to increase the likelihood of going where you want to.
You can only access it by getting a good night’s sleep. If you have a vivid dream of going to any place in one of the other zones, you are there! This is a technique known as "sleep drifting."

Any actions committed in this special kind of dream are reality, and if you bring items or people with you from that zone, for instance, they will be there with you when you awake back in your native zone.

Most people are born with travel restrictions they will never know about. In other words, they may never have dreams about reaching certain places from other zones; they simply cannot.
There are two known individuals in existence who have complete travel access with no limitations.

Travel from Zones 2 or 3 to Zone 1 is somewhat risky, because Earth’s humans do not often take well to creatures they do not understand. The safest way to transport oneself to Zone 1 is in the form of “media.” This means visiting Earth by way of television programming, radio waves, comic books and toys is the safest route, because they perceive these visits as fiction.
In other words, direct communion with Zone 1 humans is complicated and not advised.

In general, the existence of the Fluorescent Zone and, more specifically, the Hidden World within it is unknown to most Earth people. It is ancient knowledge for some.

What is a “Layer”?

If you think of a timeline of events in Zone 1—our world—in the way history books and calendars do, you perceive events as happening in a linear sequence. The 18th century leads into the 19th century, which leads into the 20th century, etc. You may think of major events like great wars, seminal artworks, changing world leaders, etc. as being signifiers of time progressing. However, all events actually exist simultaneously in separate “layers." In this logic, today is happening concurrently with 50 years ago; it is simply on another layer.
In other words, time travel is possible when one is traveling zones!

When you sleep drift to a given place from another zone, there is no guarantee that it is the “present day” version of that place. Say you are from Michigan, USA from Zone 1 in the year 2023 and you dream about being in Majonia from Zone 2. Well, you may not arrive in the present day version of Majonia that runs concurrently with Michigan, USA in 2023. You most likely will, but not necessarily.

Because the past as well as the future hold a great many things that do not exist in the present, layer traveling is also quite heavily restricted by galactic law.
Once again, those same two individuals mentioned before are the only ones with access to any layer they please, in any zone. Maybe some higher power has ordained all this?