What are “The Backrooms”?

The Backrooms are any maze-like network of rooms, hallways, or spaces that appear to lack any identifiable exit. These [mostly fictional] locations are explained as places that you “glitch” into — areas people vanish to, where one gets lost, where one can quickly go missing.

Level 1 — “Lurking Danger” — Source: BackroomsFandom

First seen on 4chan, The earliest known Backroom, also known as “Level 0,” is the most well-known of Backroom levels.