This verse is part of Joseph's interaction with his brothers in Egypt, whom he recognizes but who do not recognize him. He is testing them to see if they have truly changed since they sold him into slavery years earlier. Here's a breakdown of what the verse means:

"Send one of you, and let him get your brother...": Joseph demands that one of the brothers travel back to Canaan to retrieve their youngest brother, Benjamin. This is crucial because:

Benjamin was the only other son of Jacob and Rachel (Joseph's full brother). By demanding Benjamin, Joseph is checking on Jacob's well-being and the relationship between the brothers.
It forces them to return to their father with this strange and difficult request, putting them in a position of vulnerability and testing their honesty.

"...and you shall be bound, that your words may be tested, whether there is truth in you...": The remaining brothers would be held as prisoners in Egypt until the brother returned with Benjamin. This ensures they would follow through with their promise and would give Joseph time to assess their behavior and motives. Joseph claims that this test is to determine if their story is truthful.

"...or else by the life of Pharaoh surely you are spies.": Joseph accuses them of being spies, a serious charge. The phrase "by the life of Pharaoh" is a common oath in Egyptian culture. It emphasizes the gravity of his accusation. If they are lying about their family and purpose in Egypt, he threatens them with the punishment due spies.

In essence, Joseph is putting his brothers in a difficult position to assess their character and determine if they have changed since the time they sold him into slavery. He wants to know if they are still deceitful and cruel, or if they have become honest and caring. He frames his demand as a test of their truthfulness to justify his harsh treatment, but his true motives are likely a mix of justice, revenge, and a desire to understand his family.