What does Genesis 42:20 mean?

Joseph was sold into slavery by his own brothers (Genesis 37:28), only to rise to power as the governor of all Egypt (Genesis 41:44). During the famine predicted by Pharaoh's dreams (Genesis 41:28–32), Joseph's brothers have come to buy grain (Genesis 42:1–5). They don't recognize Joseph, who speaks harshly to them, partly to maintain the secret (Genesis 42:6–17). After holding them for three days, he has told his ten older half-brothers that he will allow them to live if they do as he says. Nine of them must travel back to Canaan with food for their families. One must stay behind as a kind of collateral (Genesis 42:18–19).

In the moment, Joseph's actions seem vindictive. Later verses, however, show the compassion he has for his family, despite their betrayal. This is a way for Joseph to test his brothers, while giving them a well-deserved rebuke, and to secure his family against starvation (Genesis 47:11–12).

Now Joseph gets to the one point on which he insists: They must return with the youngest brother to prove they have told the truth. If they come back without the boy, he will have them killed. If they never come back, it will mean abandoning their brother forever. The brothers agree to this immediately, but a difficult discussion about the plan will follow.