From what I know about the Backrooms it's more of a Fridge Horror type of CreepyPasta.
Basically the idea, as far as I know, is that the world itself is more like a cross dimentional setpiece and the 'backrooms' are the 'backstage' of that set so to speak. How the Creepypasta generally goes is that someone steps through a doorway or an elevator or something and instead of coming out where they are supposed to they end up in The Backrooms, which is nothing but an endless maze of empty corridors, rooms and hallways that seem to lead on forever.
Essentially the creepyness factor of the Backrooms comes from the idea that no one knows what they are for, why they exist or exactly how to get there. You just kinda 'end up there' on accident due to some weird glitch in the system, and no matter how many people end up in The Backrooms, they are always completely empty and devoid of life when you get there.
I think that's the gist of it anyway.