What does Acts 19:21 mean?

Paul has been in Ephesus for a few years. He stays in the city because he seems to prefer not to visit churches another missionary has planted—likely to avoid interference. Epaphras carried the gospel to nearby Colossae, Laodicea, and Hierapolis (Colossians 1:7–8; 2:1; 4:12–14). Although Apollos, Priscilla, and Aquila had taught about Jesus in Ephesus before Paul arrived for his extended stay (Acts 18:24–28), the church didn't start until three months after Paul arrived—when the synagogue kicked him out (Acts 19:8–9). During his second missionary journey, Paul had planted churches in Macedonia—Philippi, Thessalonica, and Berea—and in Corinth in the province of Achaia in Greece (Acts 17—18). He really wants to visit Rome and Spain (Romans 15:23–24), but knows he needs to return to Macedonia and Achaia and build up the churches there. He also needs to collect money for the church in Jerusalem, then take the money to James and the apostles (1 Corinthians 16:1–4). Rome will have to wait.

Paul's not quite ready to leave Ephesus, however—partially because his relationship with the church in Corinth is going through a rough patch.

First, he sends Timothy and Erastus to Macedonia (Acts 19:22). He plans to sail directly to Corinth, then travel north to Macedonia. But he hears things about the church in Corinth that make him livid. Possibly, it's that the church has split up into rival sects: some following Paul, some Apollos, some Peter, and some Jesus. Also, a man in the congregation is openly engaging in heinous sin while not being disciplined by the church (1 Corinthians 1:10–17; 3:1–23; 5:1–8). Paul can't visit them; he's just too emotional (2 Corinthians 1:15–16, 23; 2:1–4). He writes them a scathing letter and possibly sends it with Titus (2 Corinthians 7:6–7, 13).

After Paul sends the letter, he gets anxious about the Corinthians' reaction to it, even regretting that he'd sent it (2 Corinthians 7:8). He's already decided to go north to Troas and then west to Macedonia (1 Corinthians 16:5), but when he doesn't find Titus in Troas his anxiety grows. He leaves Troas for Macedonia, surrounded by spiritual warfare and filled with anxiety. Thankfully, Titus is there and brings Paul encouraging news: the church in Corinth longs for Paul and is "grieved into repenting" (2 Corinthians 2:12–13; 7:5–9). Paul travels through Macedonia and down to Corinth where he stays for three months (Acts 20:2–3).

From Corinth, Paul plans to sail straight to Caesarea Maritima in Judea and travel to Jerusalem. Unfortunately, the Jewish leaders of the synagogue plot against him, and he must go back north through Macedonia, then east to Troas (Acts 20:3, 5–6). Shortly after Paul arrives in Jerusalem, Jews from the area around Ephesus falsely accuse him of taking a Gentile into the temple. Paul spends two years under house arrest before being taken as a prisoner to Rome.