What does Genesis 44:5 mean?

Joseph has commanded his steward to chase down his 11 brothers, already on their way back to Canaan (Genesis 44:1–4). When the steward catches up to them, he is to speak to them as if they've done something wrong. The servant was told to specifically ask about a silver cup. They are unaware it has been placed in Benjamin's sack by the steward himself, on Joseph's orders. They also have no idea that Joseph is Joseph (Genesis 42:8); they have not yet recognized the powerful Egyptian governor as their estranged brother.

The steward must emphasize that they have done evil in stealing the cup. He is to emphasize this cup is the one the Egyptian ruler drinks from. It is a personal item, and one he uses for "divination." Divination is the practice of using materials—such as liquids, bones, dice, cards, or animal organs—to supernaturally seek out information. This has been a common practice in most cultures throughout history. Since it seeks to uncover information God has not provided, and can put the diviner in contact with demons, it would be strictly forbidden by God's Law to the Israelites (Deuteronomy 18:10).

It's extremely unlikely that Joseph truly practiced divination. The One, true God Himself revealed things to Joseph without the use of any kind of apparatus (Genesis 37:5–9; 41:25–28). Most likely, Joseph told the steward to mention divination to further his brothers' assumption that he was an Egyptian lord.