These are the famous last words of Jesus, repeated before leaving the world and returning to the Father.
The promised Holy Spirit would clothe the disciples with power and is now described as a baptism.
Every good Jew knew that baptism in water was what you did when you wanted to surrender your life to the plans of God. You stepped into the water and were soaked in the idea that you needed to be cleansed from shameful, godless ways.
Now Jesus describes a 2nd kind of baptism; the baptism in the Holy Spirit. The disciples were sent to wait on God for an immersion into this life of the Spirit. The effect of being soaked in God’s Spirit would be an empowerment to represent Jesus to the rest of the world.