It’s called a “Doorway Home” in-game.  You send captured beasts through it and spend some gold and you get the beasts’ anima in return.  I always called it the Soul Furnace. ;X  It’s probably what powers the Caravan and Polka just doesn’t want to explain that part.

And here’s a cute animation done for a clip from a VTuber that is just adorable: Water in the Fire, Why?