What does Acts 21:18 mean?

Paul and his traveling companions are in Jerusalem meeting with James, the half-brother of Jesus and pastor of the Jerusalem church, as well as the other elders of the church. Along with Luke, the "we" here would include Sopater, Aristarchus, Secundus, Gaius, Timothy, Tychicus, and Trophimus (Acts 20:4). These men have brought donations from their home churches in Ephesus, Troas, Philippi, Thessalonica, Berea, and Galatia for the church in Jerusalem (Romans 15:26). Paul joins the meeting as the coordinator of the support (Galatians 2:10) and to report what he has been doing for the last several years.

"To James" probably means they are meeting in James' house; there's no indication if any of the apostles are there. When the elders hear all that Paul and his team have done, the elders glorify God (Acts 21:19–20). Then they share some disturbing news.

Many years before, legalistic Jewish Christians tried to convince the churches in Galatia and Syrian Antioch that to properly worship the Jewish Messiah, Gentiles needed to follow the Jewish law (Acts 15:1). The Jews said this because they wanted to be part of the church but didn't want to lose their standing in the synagogue (Galatians 6:12–13). Paul and Barnabas disagreed and brought the matter to the church in Jerusalem. James and the elders determined that in order to make the Jews comfortable eating with Gentile believers, the Gentiles should avoid eating food sacrificed to idols and blood, and avoid sexual immorality (Acts 15:19–21).

Paul affirmed the council's decision and distributed their letter. But now someone is accusing Paul of teaching that Jewish Christians should—or must—stop following Mosaic law. This is patently false. The church comes up with a plan to prove Paul is a good Jew. In the course of fulfilling it, Paul gets arrested (Acts 21:19–36).