All the believers were meeting in one place (v.1): obedience to Jesus' command in Acts 1:8. Matt 18:20 says when two or three believers are gathered in His name, He is in the midst. The start of house churches? Similarly, in Acts 12:12, you find believers gathered in someone's house to pray for Peter (who was imprisoned by Herod). Gathering together in one place is not just significant in the "physical" sense (in that people are all in one place), but it also implies they are united in one purpose - to pray together, in one united voice.

(v14) Peter, baptized by the Holy Spirit; emboldened, speaking to a huge crowd. He quotes from Joel 2. Lord, I want to experience that same outpouring of Your Spirit.

Matthew 18:20, Acts 1:8 and Acts 2:1-21