What does Acts 7:9 mean?

Stephen is a "Hellenist," someone who is highly influenced by Greek culture. It's likely he was at the synagogue of the Freedmen (Acts 6:9) because he is not from Judea and he can reach Greek-speaking Jews there with the good news about Jesus. Even for Jews who lived throughout the Roman Empire, Judea, Jerusalem, and the temple are very important. They are the center of their identity as Jews. Stephen probably found Christ because he made the voyage to Jerusalem to worship at the temple.

So, Stephen understands his accusers' veneration of the temple. He values the temple, and he still identifies as a Jew. He would never blaspheme the temple, as his accusers have claimed. But he more fully understands that following God is more about faith than place. And of all the patriarchs, it was Joseph who exemplified faith in God despite situation or location.

The "patriarchs," here, aren't Abraham, Isaac, and Jacob, rather this refers to Jacob's sons. Specifically, those sons born before Joseph, so not including Benjamin or Joseph's two sons whom Jacob adopted: Manasseh and Ephraim. Joseph's older brothers, jealous of Jacob's regard toward him, sold him to slave traders and told Jacob a wild animal had killed him. The slavers sold him to a nobleman in Egypt. Despite Joseph's time in slavery and then prison in Egypt, God was with him. God blessed Joseph with training, wisdom, and positions of power and responsibility. While Jacob mourned, Joseph's brothers had no idea God would use their sin to rescue them and the surrounding nations from famine. This part of their story is in Genesis 37.