Paul and Barnabas got on a ship at Attalia, and they went back to Antioch, in Syria country. Antioch was the city where they started from, some time before. It was those Antioch Christians that asked God to look after Paul and Barnabas and be good to them, and then they sent them out to tell people about Jesus. That is the reason why Paul and Barnabas went back to Antioch after they finished that work.