The floor was dirt. Only the wealthy had something other than dirt.
Hence the saying, “Dirt poor.” The wealthy had slate floors that would get slippery in the winter when wet, so they spread thresh (straw) on the floor to help keep their footing. As the winter wore on, they added more thresh until, when you opened the door, it would all start slipping outside. A piece of wood was placed in the entrance way.
Hence: a threshold.