Were Priscilla and Aquila able to teach Apollos something he didn't know?

Yes. Priscilla and Aquila took Apollos aside and expounded unto him the way of God more perfectly. Apollos knew only the Gospel of the Kingdom—repentance, baptism, and believing Jesus was the Christ—but he knew nothing of Paul's Gospel, so the two tentmakers could teach him what he did not know.

Les explains that Apollos was a great speaker who knew only the Gospel of the Kingdom preached by Jesus, the twelve, and John the Baptist—repentance, baptism, and believing Jesus was the Christ. He knew nothing of Paul's Gospel. God had the right person at the right place at the right time: Priscilla and Aquila, lowly tentmakers, took Apollos aside and expounded the way of God more perfectly. They were in a position to tell him something he did not know, and Apollos, admirably, listened.

What did Priscilla and Aquila do for Apollos, and why could they do it?

Priscilla and Aquila took Apollos aside and expounded unto him the way of God more perfectly. They could do this because they were in a position to tell Apollos something he did not know, having received knowledge that he lacked.