Acts is shorthand for The Acts of the Apostles. I share from the English Standard Version’s introductory text, which describes it more eloquently and succinctly than I can:
“Acts picks up where Luke’s Gospel leaves off, recording the early progress of the gospel as Jesus’ disciples took it from Jerusalem throughout Judea, Samaria, and the rest of the Mediterranean world. The story begins with Christ’s ascension and the events of Pentecost. As Gentiles begin responding to the gospel, the focus shifts to Paul and his missionary journeys. Acts forms a bridge between the four Gospels and the rest of the New Testament, showing how the apostles carried on Christ’s work and providing a historical background for Romans through Revelation. The Acts of the Apostles is the second of two New Testament books written by Luke. Like his Gospel, Acts was a letter to Luke’s friend Theophilus, written sometime in a.d. 62–64.”
Those last sentences give us the reasons we are dealing with this as one book in two volumes (i.e. “Luke-Acts”); it will also become clear from the text itself as we get into Luke’s prologue to the book of Acts.