In the first book, O Theophilus, I have dealt with all that Jesus began to do and teach, 2 until the day when he was taken up, after he had given commands through the Holy Spirit to the apostles whom he had chosen. 3 He presented himself alive to them after his suffering by many proofs, appearing to them during forty days and speaking about the kingdom of God. (Acts 1:1-3)
The Acts of the Apostles is the history told by the physician Dr Luke of how faith in the Saviour Yeshu Ma Shi Ga spread first in Israel and then to other parts of the world. Luke wrote both like a journalist, and in very scholarly language; he was careful in recording facts and detail.
His story is told to a believer called Theophilus. And to this man he had already written the gospel story in another book called The Gospel of Luke, in much the same way as Mark had told the same good news to the people living in Rome.
The first three verses are a reminder of how Yeshu made appearances in his risen body to the close disciples and to others, and taught them over a period of time before being carried up into heaven.