What does Acts 18:21 mean?

Paul is headed home after a long second missionary journey. He and Silas started by revisiting the churches Paul and Barnabas had planted in the province of Galatia in central modern-day Turkey. They picked up Timothy in Lystra and traveled west to Troas where they met Luke. God sent Paul a vision to come to Macedonia, so they crossed the Aegean Sea and planted churches in Philippi, Thessalonica, and Berea before Paul had to flee to Athens to escape persecution. From Athens, he went west to Corinth where he met Priscilla and Aquila. He was partway through his eighteen-month stay when Silas and Timothy rejoined him. After a tense standoff with irritated Jews and an indifferent proconsul, Paul, Priscilla, and Aquila crossed the Aegean Sea east to Ephesus (Acts 15:40—18:19).

In Ephesus, Paul visits the synagogue, as usual, and explains how Jesus of Nazareth fulfills the prophecies of the Messiah given in the Hebrew Scriptures (Acts 18:19–20). The attendees are interested and want him to stay longer, but he is apparently anxious to reach Jerusalem. Possibly because he needs to fulfill the requirements of a Nazirite vow (Acts 18:18) or, perhaps, to be there for Passover.

He will return, however. On his third missionary journey, he will spend three years there, building up the church. He will be so successful, and so many Gentiles will respond to Jesus' offer of forgiveness, that the craftsmen who make idols for Artemis worship will find their business greatly diminished (Acts 19:17–41). On his way back to Jerusalem, He will meet the Ephesian church elders in nearby Miletus. He will encourage them and tell them he will not see them again. The elders weep and pray, and accompany him to his ship (Acts 20:17–38).