Apollos knew of the forerunner (John the Baptist and his baptism of repentance), but did not know that Messiah had come. He knew the way (the path, the conduct), the water baptism (repent, expect) but did not know Messiah.
This man [Apollos] was instructed in the way of the Lord; and being fervent in the spirit, he spake and taught diligently the things of the Lord, knowing only the baptism of John.
When Aquila and Priscilla hear him speak, they instruct him more fully. After that, Apollos begins to show that Jesus, who had already come, was Christ.
For he mightily convinced the Jews, and that publickly, shewing by the scriptures that Jesus was Christ.
Before that instruction, Apollos was teaching a works-based way, but once he learned of Jesus Christ, he taught the message of grace.
And when he [Apollos] was disposed to pass into Achaia, the brethren wrote, exhorting the disciples to receive him: who, when he was come, helped them much which had believed through grace:
In the next chapter, Paul encounters some men who had been baptized by Apollos into the baptism of repentance, not knowing that Messiah had come. This confirms what happened in chapter 18; that is, Apollos knew of the forerunner, but not Jesus Christ.
Then said Paul, John verily baptized with the baptism of repentance, saying unto the people, that they should believe on him which should come after him, that is, on Christ Jesus.