What does Acts 2:16 mean?

There are about 900,000 visiting Jews in Jerusalem from as far away as Rome, Alexandria, and Mesopotamia (Acts 2:9–11). A crowd of them have just heard a group of Galileans speaking in their native dialects (Acts 2:8). The question floating around the crowd is "What does this mean?" (Acts 2:12). Peter is more than happy to explain.

We don't know much about Joel except that he was a prophet in Judah around 835 to 800 BC. The prophecies in his book cover events immediate to his time, including the devastation wrought by a horde of locusts, but also the beginning years of the church and the end times.

In the time of Joel, a huge swarm of locusts destroyed the crops, leaving Judah in famine (Joel 1:1–12). Joel likened the locusts to God's wrath that would fall on His people if they didn't repent (Joel 1:13–20). If they did repent, God would have pity on them (Joel 2:18–27). As a sign of His reconciliation with His people, He would send the Holy Spirit so that the people would prophesy and see visions (Joel 2:28–29). But not everyone would return to God; those who refused God would still see His judgment (Joel 2:30–31).

Peter applies this section of Joel's prophecy (Acts 2:17–21) to the experience of the audience and then immediately goes into why his listeners need to repent: they killed their own Messiah (Acts 2:22–36). His message is effective; three thousand repent and are baptized (Acts 2:41).