Apollos we are told in the preceding verses was a learned man, well educated and a Jewish believer. Further he could teach about Jesus, and do it with great energy. But it wasn't enough - something was missing, it was not all God intended. Priscilla & Aquila saw that he only knew the baptism of John, and had greater things of God to introduce him to. They took him aside and rehearsed afresh the story of Jesus.
While this was happening with Apollos, Paul met others on his travels. Directly he asks them 'Did you receive the Holy Spirit?'. They look vacant, having not even heard of the Holy Spirit! These men also only knew of the baptism of John, a baptism preparing themselves for God.
Backtrack 100s of years to Noah and Abraham etc. God made promises to them, covenants, that Abraham's descendants would become a blessing to many. He gave Abraham a sign, a mark of the covenant promise: male circumcision to show that they belong.
Now overall it didn't work out so well, because our capacity to ignore God and do our own thing is so great which builds a wall between us and God. A wall that even if we try we cannot dismantle. But God can - something that John the Baptist anticipated in months just ahead of Jesus' ministry. John the Baptist called people to prepare themselves and get ready for God. Now in Israel they had developed a practice of ritual washing, cleansing yourself from impurity to be ready - so it was natural for John to dip people in water to become ready: the baptism of John.
But John knew there was nothing special in the murky water of the river Jordan. He knew it was all about the one who was to come, whom he pointed to: Jesus. Jesus would take down the wall, Jesus would truly re-unite - so when Jesus appeared John was glad to hand over and let people be baptised by Jesus (or his followers): the baptism of Jesus.
But there was more to it. Way back with Noah/Abraham God set a deal that He would be resident among a people who belonged to Him, a people who could witness to the world. Now in Jesus we have God in the flesh among the people. Through his life, death and resurrection God wanted to remix that deal to a whole new level. God would be resident not just among people but in each believer - God resident in the lives of each and every person following Jesus.
This is the baptism of the Holy Spirit - something John the Baptist told about ('he will baptise you with the Spirit'), something that Jesus instructed his followers to wait for when he physically departed from them. That is why for guys like Apollos and the others Paul met, who only knew the baptism of John, it was obvious that they needed a whole further deal in their discipleship - namely following Jesus and receiving the Holy Spirit.
This is about transformation not information. Apollos had plenty of information but needed the Spirit's transformation. Similarly for the guys Paul met - discipleship is not simply amassing more info about God, but being more and more inhabited by God. On a discipleship journey we will want to get rid of anything that blocks the Holy Spirit!
With this filling Apollos wanted to venture out, and the men started prophesying - telling the wonders of God. The Spirit brings a natural 'OUT' component to our lives, hence the key question:
[OUT] Where is God to empowering you to minister?
Challenge: Seek to be ever closer to Jesus → He will send you
You will be aware of situations that need change. You yourself cannot bring that change ... but with the Holy Spirit in you things are possible. Let us learn to minister in the power of the Spirit.
Of course the outward call typically induces feelings in us of 'yikes, I suppose I ought' etc. The danger is to then summon up the energy to go out - but that is not what we are after. Rather it is your desire to be in His presence, close to Jesus. When you seek His presence, you allow His Spirit to come, and for Him to empower you, which will bring a natural desire & ability to reach out. Hence the challenge above - adopt one of our discipleship habits 'Ever Closer to Jesus'. You will discover He will send you ...
Finally another thing the filling of the Holy Spirit does is to confirm to the believer that they belong. A natrual way for you to communicate to your children that they belong is to give them a hug. Many on receiving the Spirit talk of a hug from God, confirming that they belong. This takes us full circle back to covenant promise, with the filling of the Spirit also being a covenant sign of belonging.
So be filled, and as Paul writes, go on being filled.