During the Feast of Pentecost, 120 of Jesus’ disciples had gathered in an upper room in Jerusalem, praying and waiting for the power of God to fall on them. Jesus told them to wait “until you have been clothed with power from on high.” (Luke 24:49 NIV) On the day of Pentecost, the power of God fell on them. They received the baptism of the Holy Spirit and power. (See Acts 2)

They came down from the upper room and onto the streets praising and worshiping God in languages they had never learned.

Onlookers were shocked. Acts 2:7-8, “Utterly amazed, they asked: ‘Aren’t all these who are speaking Galileans? 8 Then how is it that each of us hears them in our native language?’”

The 120 from the upper room were mostly simple, uneducated people. They knew Hebrew and spoke a Galilean dialect of Aramaic in everyday life. How would they know the languages of people from all over the known world?'

The Jewish people from all over the Middle East who had traveled to Jerusalem for the Feast of Pentecost, were amazed when they heard the local people speaking their languages. Persian dialects, Coptic (Egyptian), Latin, Greek and many other languages from all over the known world.
A huge crowd formed. And the Bible records this question from the crowd:
Acts 2:12 NIV

12 Amazed and perplexed, they asked one another, “What does this mean?”
This is the first great question we need to ask when we don’t understand. “What does this mean?”

Why did God do this? Because He wanted to get the attention of the crowds.
This is what God does. God creates a holy curiosity in us. It is an invitation to come closer to Him

As Peter spoke to the Jewish faithful from all over the world, God opened the scriptures to them. Peter showed them how Joel and David prophesied about Jesus. And Peter confirmed that Jesus is the Christ – the Messiah they had been waiting on for so long.

The Holy Spirit revealed the truth to the crowd, and many were convicted in their hearts. They felt compelled to do something about what God had just revealed to them.

Acts 2:37
When they heard this, they were pierced to the heart and said to Peter and the rest of the apostles, “Brothers, what should we do?”
This is the 2nd great question after God reveals meaning and truth to us: What should I do?
Revelation from God is an invitation to do something with God.

Acts 2:38-41
38 Peter replied, “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. 39 The promise is for you and your children and for all who are far off—for all whom the Lord our God will call.”
40 With many other words he warned them; and he pleaded with them, “Save yourselves from this corrupt generation.”

And 3,000 people took action that day!
Verse 41 Those who accepted his message were baptized, and about three thousand were added to their number that day.