Beached things (BTs) are entities whose souls are stranded in the world of the living following the events of the Death Stranding. BTs are considered extremely dangerous and are almost invariably hostile toward living things.

Because the bodies of BTs contain antimatter, if they consume a living thing, a voidout will be triggered, causing a massive explosion capable of destroying entire cities. A beached thing comes into existence when the deceased soul of a living thing fails to cross through the Beach into the world of the dead, consequently becoming "stranded" in the world of the living, with their connection to the other side maintained through an umbilical cord.

The Death Stranding occurred concurrently with the first recorded sighting of a BT by a doctor performing a Caesarean section on a brain dead mother. Due to the Death Stranding, when a human being dies, their "ka" (soul) will attempt to return to their "ha" (body) after death. The only way to prevent this is to destroy the body, usually through cremation at an incinerator; if this is not carried out, the ka will return to the ha and cause a voidout due to the combination of matter in the ha and antimatter in the ka.

The ka will then become a BT, stranded in the world of the living. BTs can be perceived by individuals suffering from DOOMS, or via a trance connection to bridge babies, which exist between the worlds of the living and dead. The extent to which DOOMS sufferers can interact with BTs varies. Sam is capable of sensing BTs, but cannot see them without the assistance of Lou.

Fragile, meanwhile, is capable of seeing BTs without aid. With a DOOMS level of seven at minimum, Higgs is capable of not only seeing BTs, but also controlling them at will.

Source: https://deathstranding.fandom.com/wiki/Beached_thing