What does Acts 18:25 mean?

Apollos is a Jew from the city of Alexandria on the Egyptian coast. He is eloquent, competent in the Scriptures, fervent, and a good, accurate teacher (Acts 18:24). He has come to Ephesus with a full understanding of John the Baptist's call to repent (Mark 1:2–8), but only nominal information about Jesus.

It's not clear what Apollos doesn't know. By this time, he must know about the crucifixion and resurrection, but not about being baptized in Jesus' name. That would indicate he's somewhere between the repentance that John preached (Mark 1:4) and the comprehensive forgiveness of sin and reconciliation to God that Jesus provides. He must know Jesus is the Messiah because that's what John preached (John 1:19–34), but he probably doesn't realize how big a break Jesus-followers made from Judaism. He doesn't have the Holy Spirit and seems to share beliefs with others in Ephesus (Acts 19:1–6).

When the synagogue leadership in Ephesus invite him to speak, Priscilla and Aquila are there. The couple are refugees from Rome who learned about Jesus from Paul in Corinth (Acts 18:1–3). When Paul left Corinth, he brought the two and left them to continue the work he'd barely started (Acts 18:18–19). Priscilla and Aquila pull Apollos aside and explain to him what John meant when he said, "I baptize you with water, but he who is mightier than I is coming, the strap of whose sandals I am not worthy to untie. He will baptize you with the Holy Spirit and fire" (Luke 3:16). Apollos believes and goes on to build up the church Paul planted in Corinth (Acts 18:27–28).