After Abraham had sent his son Ishmael away to live with his mother Hagar, Ishmael settled in the wilderness and married a Moabite wife. Years passed. Abraham wanted to see how his son was faring, but he did not identify himself. He came to the tent disguised as a traveler, asking for water and shelter.
Ishmael's wife was home alone. She looked at the dusty old man standing at her door and decided he was not worth her trouble. She refused to bring him water. She refused him bread. She did not recognize that the stranger was her own father-in-law. Abraham left the tent without introducing himself. Before he went, he gave her a message for her husband. Tell Ishmael, he said, that an old man came by, and told him to change the threshold of his house.
When Ishmael came home and heard the message, he understood immediately. He knew his father's voice. He knew what a "threshold" meant in that voice. He divorced the wife who had shown no hospitality, and he married another woman.
Some time later, Abraham returned a second time. Again he was a stranger at the door. The new wife welcomed him. She brought water to wash his feet. She brought bread. She spoke gently. Abraham blessed her and commended her to her husband, though Ishmael by now understood that his father had been quietly inspecting his household.
The Exempla, preserving a midrash known from Pirkei deRabbi Eliezer chapter 30, uses the word "threshold" as a euphemism for the wife herself. A household's hospitality is not a decoration. It is the threshold over which every visitor must pass. Abraham, the great host of Genesis 18, could not abide a son whose doorway was shut to the stranger. Change the threshold, he said, and Ishmael understood.