Our passage starts with a few travel details. I want to highlight one detail of that because it will be important later. Barnabas and Saul (Paul) did some of their journeys with a man named John also called Mark. He was with them in Jerusalem and in Salamis. In this reading, he goes his own way, back to Jerusalem. The passage doesn't say anything about why, nor does it mention any disagreement, but Paul and Barnabas will later have conflict in their own relationship centered on this moment.
The passage overall is a great snapshot from Paul's missionary work. He often started his work in a new town at the synagogue, because Christianity was a movement within Judaism. So in the synagogue, Paul knew he would find people who already knew God's story. Like we've seen with Stephen and other sermons in Acts, Paul starts the message about Jesus by reviewing the story of Israel and God. The more rooted we are in the whole story, the more each part, including the day to day decisions we make, make sense.