It's basically an underground space where people can live in case of emergency. In theory this whole complex is a bunker, but usually they're a lot smaller, just a room to hide in during bombing raids or a back up command centre for the military to operate from safely. Nuclear bunkers from the Cold War era at least attempt to supply longer term needs because of the risk of radiation making the surface unsafe, but considering how much storage space you need for that, most of them fail pretty badly unless they were built by the government or something with a similarly large budget.