What does Acts 2:3 mean?

About one hundred and twenty Jesus-followers (Acts 1:15) are sitting in a house in Jerusalem when the Holy Spirit comes (Acts 2:1–2). At first, they hear "a sound like a mighty rushing wind" (Acts 2:2). Now bits of fire—or something with that appearance—land on them.

In the Old Testament, God often came as if in wind (Job 38:1; Ezekiel 1:4; 1 Kings 19:12), but He also came in the form of fire. He met Moses in a flaming bush (Exodus 3:2–5) and led the Israelites through the wilderness through a pillar of cloud and a pillar of fire (Exodus 13:21–22). When God met Moses on Mt. Sinai, He "descended on it in fire" (Exodus 19:18) and "the appearance of the glory of the LORD was like a devouring fire on the top of the mountain" (Exodus 24:17).

John the Baptist promised that Jesus' followers would be baptized by the Spirit, but he also said they would be baptized with fire (Luke 3:16; Matthew 3:11). The Bible uses fire as a metaphorical method of purification. Fire purified Isaiah's unclean lips (Isaiah 6:4–8) and burns away the useless chaff of the wheat (Luke 3:17). When we will be judged, our works will be tested as if they were put through fire to destroy the useless and reveal the good (1 Corinthians 3:12–15).

In this case, it appears the tongues of fire are visual manifestations of the Holy Spirit's intent. His plan is to equip the Jesus-followers with the ability to speak other languages (Acts 2:4). Jerusalem is filled with Jews from all over the Roman Empire and beyond who have come for the Feast of Pentecost. It's possible that not all of them speak Aramaic or Greek. In order to effectively spread the message that Jesus is the Jewish Messiah who can reconcile sinners with God, the people will need to hear that message in their own language. And so, the Holy Spirit makes that possible