What does Acts 8:26 mean?

Luke promised Theophilus that his first letter, which we know as the Gospel of Luke, was an "orderly account" of the eyewitness testimony of Jesus' story (Luke 1:1–4). It's reasonable to assume Luke continued this goal with this second letter, the book of Acts. "Orderly" does not necessarily mean strictly chronological. In the Old Testament, the stories of minor characters are often succinctly given before the accounts of the major figures: Cain's line is described before Abel's (Genesis 4:17–26) and Esau's descendants are lumped together in Genesis 36 before the narrative continues with his brother Jacob. This is a way of being thorough without continually interrupting the main story.

It's possible that Luke did the same here. We don't know where the account of the Ethiopian official fits with Paul's conversion (Acts 9) or Peter's encounter with Cornelius' household (Acts 10). But placing the story here completes the main story of Philip and introduces the evangelism of the Gentiles—which Peter affirms, Paul carries out, and the rest of the book records. The story of Philip and the Ethiopian official may have happened any time before Acts 21:8. Philip is not mentioned again until Acts 21:8–9 when Paul visits him on his way to Jerusalem.

This Philip is one of the church's first deacons (Acts 6:5). He fled Jerusalem when Saul began persecuting the church and first went north to Samaria (Acts 8:1–25). He is not the disciple Philip (Matthew 10:3) nor the brother of Herod Antipas (Matthew 14:3). He will eventually settle in Caesarea where he will have four daughters who are prophetesses (Acts 21:8–9).

This verse features an idiomatic phrase which is easily lost when translated. "South," literally, is "about noon," from the Greek term mesēmbrian. Since Israel is north of the Tropic of Cancer, at noon, the sun is always due-south. "Desert" here can mean uninhabited grassland or wilderness. Where we would say the road "goes down" from Jerusalem to Gaza because it goes south, Jerusalem is higher than Gaza, so the road literally goes downhill. Gaza was a city in the same place it is now—the modern-day Gaza Strip on the coast of the Mediterranean Sea. From Gaza is a major road that goes to Egypt, then south to Ethiopia.