It refers to the Holy Spirit descending among the apostles - just as Jesus had said it would. Passers-by initially thought the apostles must be drunk because of their behaviour, but Peter ...

The Father sent the Son at the very moment the Holy Spirit overshadowed ... send a fuller outpouring of the Spirit, who descends like tongues of fire, on the apostles (cf. Acts 2:1-4).

Peter's Basilica in Rome. The Holy Spirit's action is central to the movement of the plot. He descends on the apostles with power, inspires their preaching, tells them where to go and aids them in ...