My interpretation from the reading is that the Church will suffer persecution until the Lord comes for his church.
I felt that the souls were in heaven, and their proximity to the altar signifies that believers are willing to be sacraficed for the Lord.
They were killed for their faith and preaching their faith to others.
I believe they were asking for vengeance because the Lord promises it to all believers who are persecuted in his name. Don't know where the prayers are, but there are many prayers in the bible asking the Lord for his vengeance against the enemy.
The white robe singles out the martyrs who died for the Lord.
I believe the Lord ask them to wait, because of his mercy and love for others who will come to him at the final moment. God loves his people so much that he is giving them the opportunity to come to him before the wrath of his final judgement.
Praise be to God.
