The world has been devastated by a series of apocalypse-level events called the 'Death Stranding'. The US is left broken and divided, with the remaining population forced to live separated in underground bunkers and cities.
It's not known what the Death Stranding was or how it was caused as historical data has been fragmented/lost, but it appears to have opened a connection between our world and the world of the dead (Hades).
Creatures called BT's (Beached Things) roam the surface as a byproduct of the Death Stranding. They threaten human existence and can trigger a void-out (a giant explosion) if they successfully consume someone.
Bridges is a government initiative that aims to reconnect the remaining cities to form the United Cities of America (UCA).
A Bridge Baby (BB) is a baby whose mother seems to have died during pregnancy or birth.