"While Apollos was at Corinth, Paul took the road through the interior and arrived at Ephesus. There he found some disciples" NIV translation
Apollos (who we met in chapter 18) had been in Ephesus but had now moved on to Corinth as Paul arrived in Ephesus. Ephesus was an important commercial city as well as a hub of satanic worship. It would be in what is today known as the modern nation of Turkey.
We see here that although most of the city was filled with pagans, Paul did find some that believed in Jesus.
"and Paul asked them, 'Did you receive the Holy Spirit when you believed?' They answered, 'No, we have not even heard that there is a Holy Spirit.'" NIV translation
So, when Paul found this group of believers, he asked them if they had received the Holy Spirit and they said that they had not even heard that the Spirit existed. How sad this must have made Paul as he realized that life and faith were hollow without the power of the Holy Spirit in us. These people had knowledge of Jesus but not the power to do anything with that knowledge. We each should ask ourselves this same question;
Got the Spirit?
"So Paul asked, 'Then what baptism did you receive?' 'John's baptism,' they replied. Paul said, 'John's baptism was a baptism of repentance. He told the people to believe in the one coming after him, that is, in Jesus.'" NIV translation
So, these people only knew about repentance and not about living in power. Paul reminded them that John had called people to believe in Jesus not to follow himself.