The Backrooms is a realm that first appeared on 4Chan. This realm is out of the boundaries of the real world and can only be accessed by nocliping through a floor or wall in a place that inspires deja vu or an uncomfortable feeling of nostalgia. It consists of a never-ending maze of yellow-ish empty rooms with beige carpets, old wallpaper, the smell of mildew and moldy carpets, and the faint buzz of fluorescent lighting.

A really big yellow liminal office building that you get to by no-clipping IRL.

The Backrooms are a concept first showcased in a post on 4chan, where one user described a realm made up of nothing but infinite, empty, interconnected rooms with horrible yellow wallpaper and loud fluorescent lights. The concept has now been expanded upon and revamped numerous times over.

The Backrooms, in general, are a series of liminal spaces designed to evoke a feeling of eeriness and nostalgia. They are designed to be a form of psychological horror, with the emptiness and eerie feelings as the main point of the Backrooms. They take elements from everyday life, things that one would not ordinarily pay attention to, and takes them out of context to create something uncanny. Something empty. They are designed to feel surreal, dream-like. Often, the structure and physical design of these liminal spaces are completely nonsensical, as if they were not deliberately made, which, of course, only adds to the psychological horror of the Backrooms.

If you'd like a more in-depth explanation, take a look at Solar Sands' video on YouTube on the subject, titled, "Liminal Spaces".

There's also a Fandom site for the Backrooms, at backrooms.fandom.com.

If you're not careful and you "noclip" out of reality in the wrong areas, you'll end up in the Backrooms, where it's nothing but the stink of old moist carpet, the madness of mono-yellow, the endless background noise of fluorescent lights at maximum hum-buzz, and approximately six hundred million square miles of randomly segmented empty rooms to be trapped in
God save you if you hear something wandering around nearby because it sure as hell has heard you.

The Backrooms are an alternate dimension, but one that is not like many other alternate dimensions in the universe. While others seem to be pocket dimensions that are linked to our own reality, the Backrooms runs deep INSIDE our own reality, meaning there is no physical way to enter it. Rather, a person enters the Backrooms when in a state of deep despair, or during sleep.

The backrooms are an endless set of office cubicle styled rooms set in a plane slightly agitated from normal reality.

The action of reaching the backrooms is described as "no-clipping" out of reality but there is no natural way to do this.
People who end up in the backrooms are subject to what can only be considered a glitch in reality and cannot be called at fault.
Ending up in the backrooms happens by completely random chance and can happen to anyone.

The backrooms are considered a place that you end up when reality has corrected and irregularity while someone was inside the zone of this irregularity but the zone does not contain area in a normal space and can be compared to how fifth dimensional objects are perceived from the fourth dimension.
Though the backrooms cannot be escaped by any normal means any living being can perform, there are several different fates that can occur while wandering the backrooms.

The backrooms is a series of liminal spaces, with nearly no human interactions and paranormal entities. The backrooms consists of only 11 stable levels, and more than 3 quintillion unstable levels. Be careful, you might noclip out of reality right now. And good luck getting out of the backrooms.

The Backrooms has a high number of levels, including the sub-levels. You can get into the backrooms by noclipping into a wall, or entering a glitched wall. We currently live in the frontrooms.

Backrooms are places that exists out of our reality, the perfect example is the space behind mirror. Many entities live there for example Chaxs. Backrooms are described as endless dark space without comfort or warm. The floor is cold and dusty and there is no light. The time doesn't exist, therefore you can spend years in there desperetly begging for help.

Not to be confused with Kane Pixels The Backrooms (Found footage)
"If you're not careful and you "noclip" out of reality in the wrong areas, you'll end up in the Backrooms, where it's nothing but the stink of old moist carpet, the madness of mono-yellow, the endless background noise of fluorescent lights at maximum hum-buzz, and approximately six hundred million square miles of randomly segmented empty rooms to be trapped in

God save you if you hear something wandering around nearby because it sure as hell has heard you."

Basically the back of reality. The backrooms was first posted on 4Chan on 2019, and grew a ton of attention and people even made Fan-made Backrooms levels.

Also there's a drink called "Almond Water"
And the backrooms is an example of a "liminal space"