In Death Stranding the term BB stands for Bridge Baby (plural: BBs = Bridge Babies). A Bridge Baby is what Sam carries around in a yellow tank.
These Bridge Babies were developed by the government as equipment – they are not treated as living things. They have high DOOMs levels (supernatural powers) and allow the wearer to “connect to the other side” (afterlife / beaches) to see BTs (Beached Things – the ghostly figures).
Without a BB, the BTs would be invisible. If people were to walk into them, the would get killed. So a BB is really just a sensor upgrade. The average BB gets decommissioned (destroyed) after 1 year.
BBs are harvested from the womb of braindead women (their Stillmothers) at 28 weeks old. Growing inside a braindead mother somehow gives them special abilities.
They are “right between the world of the living and the dead”, neither alive nor beached. Deadman recommends not to get attached, he says they are just equipment.
Simply Put: BB = Bridge Baby – allows you to see ghosts