In Death Stranding the term BT means Beached Thing. They are ghosts from the afterlife that are entangled in the world of the living.
BTs (Beached Things) can only be seen by people wearing a BB (Bridge Baby), or by people with high DOOMs levels (supernatural powers). To the average human they are invisible, but their footprints can be seen as they interact with the environment.
Every BT is connected to the afterlife (= beach) by an umbilical cord. Cutting their umbilical cord returns them to the afterlife.
Basically, in Death Stranding the world of the living and the dead has become entangled and that’s why these ghostly figures are appearing.
BTs / Beached Things only show up during Timefall (rain).
Just like people can’t see a BT, the BT can’t see humans either. However, BTs are able to hear very well. If you were to run near them, they’d come after you and drag you away to their boss, a “catcher” (big goo monster). As long as you stay still they have no idea where you are and won’t attack. Once detected, you can hold your breath with 
to stay quiet and they might not find you.
It’s like playing hide and seek, except both participants are invisible to each other.
Simply Put: BT = Beached Thing – a ghost from the afterlife that’s roaming the earth.
Hartgroove says
One more thing, you can see BTs if you stay still, if you move they disappear from your sight. You can also scan them while you still.
You can also use bola gun with blood on them or any other blood weapon or just cut a cord, they drop crystals on death.
Sai says
A very s c a r y game of hide and seek