You will be baptized with the Holy Spirit.

Acts 1:1-11
Jesus taken up into the heaven.
After his suffering, he presented himself to them and gave many convincing proofs that he was alive. He appeared to them over a period of forty days and spoke about the kingdom of God.
“Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about. For John baptized with water, but in a few days you will be baptized with the Holy Spirit.”
But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.
“Men of Galilee,” they said, “why do you stand here looking into the sky? This same Jesus, who has been taken from you into heaven, will come backin the same way you have seen him go into heaven.”
The first part of Luke's work is an account of all that Jesus did and taught during his life on earth. Act continues the story after is return to God , where his work continues through the apostles empowered by the Holy Spirit at Pentecost. It shows how the promise in verse 8 was fulfilled.
A could: the sing of God's glory an the means of conveying Jesus from them.
The white clothing indicates angels.
Why?
Jesus is resurrection and was with apostles 40 days and spoke about the kingdom of God.
But apostles were still wait for Jesus may restore the country of Israel. Jesus has more power than build Jerusalem but kingdom of God on the world.
Jesus asked apostles wait be baptized with the Holy Spirit, after that they may have power to do the mission to be Jesus witnesses in Jerusalem, and all Judea and Samaria and to the ends of the earth.
This is the mission to all Christians preach gospel to all the world. let people know the kingdom of God.
Jesus will come back again. How he was going and return back with the same way by cloud.
Holy Spirit may let us know the word of the Lord and have power to be witness of Jesus.
How?
Jesus is resurrection and be his witness in my city and around the world.