When we look at an icon, especially a traditional one, we realize that it is not a naturalistic photographic image. Shapes and sizes, including human figures, often seem unnatural. The perspective is not what we are used to. Finally, time also appears distorted, with events and people of different times put together. So an icon is not like a photograph. It is more like a portrait done by a skilled artist who can often bring out depths and nuances that a camera could not catch.
This is all true of the Pentecost Icon. Pentecost was an actual historical event. We can read about it in chapter two in the Acts of the Apostles. But the icon gives us new levels of meaning than the text alone does. So for example, Saint Paul is seated among the apostles even though he was not even a Christian on the day of Pentecost.
Often the icon will show rays of light descending upon the Apostles from on high. This shows the descent of the Holy Spirit on the Apostle in tongues of fire. Sometimes the Holy Spirit will be depicted in the form of a dove. At the bottom of the icon an old king on a dark background is shown. This is Kosmos. He represents all of fallen humanity because we remember the world until the coming of Christ lay in darkness. In the center of the Apostles there is usually an empty seat. This is the seat of Jesus Christ, who has ascended into heaven but is invisibly present. Finally, sometimes Mary the Mother of Christ is present.
In conclusion we can see that this icon depicts the reality of the world, which still lay in darkness to a certain extent, but is being transfigured by God through the church as depicted here.
Troparion — Tone 8
Blessed art Thou O Christ Our God / Thou hast revealed the fishermen as most wise / By sending down upon them the Holy Spirit / Through them Thou didst draw the world into Thy net / O Lover of Man, Glory to Thee!
Kontakion — Tone 8
When the most High came down and confused the tongues, / He divided the nations; / But when he distributed the tongues of fire / He called all to unity. / Therefore, with one voice, we glorify the All-holy Spirit!