What does Acts 19:2 mean?

In Ephesus, Paul encountered twelve men who have heard the message of John the Baptist: that they need to believe in Christ and resolve to live a more righteous life. It's possible they heard this message directly from John, as Jews tried to go to Jerusalem periodically for the feasts. But they may also have learned it from Apollos, a Jewish scholar and speaker who taught about John in Ephesus before learning to follow Jesus and moving to Corinth (Acts 18:24–28).

Paul asks the twelve if they have received the Holy Spirit. If they have, it means they are Jesus-followers and Paul can proceed with discipleship. They respond that they don't know who the Holy Spirit is. Although Aquila and Priscilla have been in Ephesus for a while and host the church at some point (1 Corinthians 16:19), the understanding that God is Father, Son, and Holy Spirit is not well established. The Holy Spirit is mentioned in the Hebrew Scriptures, starting in Genesis 1:2 and going through John's ministry (John 1:33), but He isn't quite understood as a distinct Person. And, although the Holy Spirit filled people in the Old Testament, that filling was temporary. It was not the permanent indwelling of the Spirit that ensured salvation.

Paul explains to the men who Jesus is. They agree with what he says and are baptized in water. When Paul lays his hands on them, they receive the Holy Spirit (Acts 19:3–6). That doesn't mean someone has to be baptized and have hands laid on them to receive the Holy Spirit—Cornelius' household didn't (Acts 10:44–48). But the unique sequence of events provides evidence that their decision to accept Paul's teaching is the right thing to do.