“And he began to speak boldly in the synagogue: whom when Aquila and Priscilla had heard, they took him unto them, and expounded unto him the way of God more perfectly.”
King James Version (KJV)
18:26 He spake - Privately; and taught publicly. Probably he returned to live at Alexandria, soon after he had been baptized by John; and so had no opportunity of being fully acquainted with the doctrines of the Gospel, as delivered by Christ and his apostles. And explained to him the way of God more perfectly - He who knows Christ, is able to instruct even those that are mighty in the Scriptures.
Ac 18:26 Expounded unto him the way of God more perfectly. In my opinion Apollos' position was about that of the disciples of the Lord before the Great Commission was given. He had the gospel in part, but needed to be shown the way of the Lord "more perfectly". Aquila and Priscilla supplied this needed, and equipped him for gospel work.