What does Acts 7:10 mean?

Stephen continues his defense against charges that he blasphemed the temple. Here, he puts the building into proper perspective. Joseph, son of Jacob, spent years in slavery and prison in Egypt, yet God was "with him" (Acts 7:9). Stephen quickly skips over Joseph's hardships during his initial years in Egypt. He doesn't need to go over Potiphar's wife's false accusation or Joseph's ensuing years in prison (Genesis 39:1–41:36). Stephen's audience knows the stories well. Stephen is just pointing out how God was with Joseph, even though he left the land God had promised Abraham's descendants and lived his life in Egypt.

God elevated His servant and prophet despite Joseph's rejection and betrayal by his brothers. Joseph was verified as a prophet through his own dreams (Genesis 37:1–11) and by his God-given ability to interpret others' dreams (Genesis 40:1—41:36). Joseph eventually rose to be the second most powerful man in Egypt.

Stephen has been showing how Jews were God's people without a nation or a temple. Now he starts to show how their rejection of God's prophets was a rejection of Him. Despite God's obvious hand on their brother in his younger years, nine of Jacob's oldest sons had sold Joseph into slavery (Genesis 37:12–36). Although Stephen's accusers claim to honor Moses and the other prophets of Israel, the Israelites have a long history of ignoring, rejecting, and even killing prophets. They rejected Moses before he led them out of Egypt and after (Acts 7:35, 39–40), they killed many of the prophets God sent throughout their history (Acts 7:52), and they killed the Prophet Moses promised would follow him: Jesus (Acts 7:37, 52).