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.