What does Acts 2:5 mean?

It is the first Pentecost in Jerusalem since Jesus' crucifixion and resurrection. Days ago, He ascended into heaven (Acts 1:9). Moments ago, His followers received the Holy Spirit (Acts 2:1–4) as He had promised (John 16:7). They are now, suddenly, speaking several different languages they've never learned.

During the three main Jewish feasts—Passover, Pentecost, and the Feast of Tabernacles—Jerusalem's population of 100,000 is joined by about 900,000 Jews from different countries (Acts 2:9–11). In addition, it is custom, if one can manage it, for elderly Jews to emigrate to Jerusalem so they can die on Jewish soil. And so, Jerusalem is filled with synagogues that cater to specific groups, whether Freedmen, Cyrenians, Alexandrians, or Cilicians (Acts 6:9). Some speak Aramaic, some speak Greek, but one of the reasons for the different synagogues is so they can communicate freely in the regional language of their upbringing.

"Devout" is from the Greek root word eulebēs; it means "pious and dutiful." This covers religiously observant Jews as well as proselytes (Acts 2:11). A proselyte is a Gentile who worships the Jewish God in the Jewish way, to the point that he agrees to become circumcised. When applied to a Gentile, "devout" or "righteous" may mean he follows Judaism but is not circumcised.