Acts 15:21 explains that in every city, there are people who teach about Moses, because his writings are read aloud in the synagogues every Sabbath. This means that Jewish teachings and the Law of Moses are widely known and regularly taught wherever there are Jewish communities.
James is speaking to the council in Jerusalem, addressing both Jewish and non-Jewish followers of Jesus. He points out that the Law of Moses (the first five books of the Bible) is already being taught every week in synagogues, so Gentiles will have many chances to hear and learn about these teachings if they choose. This helps explain why the council decided not to require Gentile believers to fully adopt all Jewish laws right away.