As name suggests it’s a bunker that you can drive into to avoid getting incinerated by the core explosion.
Basically it would be a structure inside the mountains, accessed by some large double doors that open when a core meltdown begins. Serving mainly as a backup option if you cant reach the heli in time or missed it, the doors remain open until the last second before the actual explosion to which they immediately close at the same moment as the core explodes.
There is however a price still, you dont get counted as a survivor like those that made it on the helicopter and only get half the reward you would have gotten if you made it to the helicopter, but still earn some money for surviving the blast and from your vehicle getting destroyed unless you drove it into the bunker.