The 10 days between Ascension and Pentecost are easily skipped over because it seems like old news. It's the same old upper room, the same old players (less Judas). It's a recitation of what we already know about Judas. The new component is the election of Matthias (far right tall blue candle) to fill out the compliment of 11 to make 12. What's also new is Peter's leadership.
As I lived with this text, I realized that it is a vital pause (concept comes from theologian Karl Barth). It's a pause between one era of Jesus' physical leadership among them to the new era of Pentecost (next week) and the indwelling Holy Spirit.
This group of believers (120+) obeys Jesus, goes back to Jerusalem and waits. While waiting together in the upper room for 10 days they pray together. The word for together means "in one accord" and with deep unity. This is something new; a unity without Jesus being the controlling presence. They wait, they worship, they turn to Scripture (the Psalms) and then they agree with Peter to move to replace Judas' apostle-position with Matthais.
These are the baby steps of the church, obedience, worship, scripture, and boldness. So, what's next for you?