The backrooms is an internet myth that started out as a piece of Creepypasta on 4chan, and has since grown into a hugely popular meme with numerous spinoffs. It is a maze of hallways and rooms that appear to be randomly generated, with drab, monochromatic walls, musty carpeting and humming, buzzing fluorescent lights. The rooms are almost always unfurnished, and there is usually no way to get out.
Levels in the backrooms often connect to one another, sometimes eerily - a doorway on Level 9 might suddenly open to a room on level 57. There is also a variety of creatures that inhabit the backrooms, from the mundane to the downright frightening. Many of these entities have been documented in the wiki by the M.E.G, but there are countless other canons of the backrooms to explore, ranging from horror movies to birthday party games.
A weapon is absolutely essential in the backrooms, both for defense against creatures and other players. It is also recommended to carry a first aid kit, with supplies like bandages and antiseptic spray, as injuries are common in the backrooms. Finally, it is important to keep track of time; in the backrooms, what seems like an instant may actually be a lifetime, and vice versa.
If you want to make your own backrooms game, Kane Pixel has made a great starter pack that is free to download from ArtStation, and works with both Blender Cycles and Eevee. It is a little dated now, but should give you a good idea of the basics.