Alexandria was the second-largest city in the Roman Empire. It was known for its strong tradition of public speaking and for the writings of Philo, a Jewish philosopher.
Apollos was a skilled speaker and knew the Scriptures (the Old Testament) very well. However, his understanding of Jesus and the Holy Spirit was incomplete. He did not yet understand that believers can experience the Holy Spirit’s power in their lives now.
Priscilla and Aquila kindly took him aside and explained the way of God more accurately.