The Backrooms are NOT a horror game. It is made up of a bunch of levels that you traverse and is like an alternate reality to the real world. It is a work of fiction. That's not to say that there aren't games based off the backrooms.
Security levels show how safe a level is, 0 being the safest and 5 being the deadliest, along with other, more uncommon levels too. There are items found across the backrooms, some help and some do the opposite. entities exist.