The meaning is, that at the distance of 20 cubits, measured along the side walls of the house from the end wall, Solomon constructed a partition, which reached from the floor to the ceiling and had a doorway in it. He thus made within the house, a sanctuary for a holy of holies.
Related Commentaries of 1 Kings 6:16
1 Kings 6:16
16 And he built twenty cubits on the sides of the house, both the floor and the walls with boards of cedar: he even built them for it within, even for the oracle, even for the most holy place.