What does Acts 14:21 mean?

"That city" is Derbe, a town south of center of modern-day Asia Minor (Acts 14:20). This is the farthest Paul and Barnabas go on their missionary journey (Acts 13:1–3). It's also very close to Paul's hometown of Tarsus. They could just cross the mountain pass and take a short boat ride back to Syrian Antioch.

No one will blame them if they take the shortcut. In Pisidian Antioch—different from Syrian Antioch, where they have their base—the local leaders drove them off (Acts 13:50). In Iconium, the pair fled when they heard a mob was planning on stoning them (Acts 14:5–6). And in Lystra, the local people first tried to offer them sacrifices, thinking Barnabas was Zeus and Paul was Hermes, and then stoned Paul and left him for dead (Acts 14:8–19).

Paul and Barnabas cannot leave without revisiting their new friends. They know that preaching the gospel and making converts is not enough to establish strong local churches. The new believers need leadership, more training, and answers to their questions (Acts 14:22–23). Paul will visit these churches again on his second journey, meeting Timothy in Derbe of Lystra (Acts 16:1–6), and his third (Acts 18:23). He'll also write the book of Galatians to make sure the Gentiles understand they are saved without following the Jewish law. Paul's job is to establish the churches well; it's the Holy Spirit's work to keep them.