There is an episode that occurs before this pericope which sheds light on Peter's question about forgiveness.
In 16:13-19, Peter surprisingly declares that he believes Jesus to be the Messiah. In response, Jesus calls Peter the rock and promises to build the church upon his leadership. It is helpful to remember that episode in which Peter is identified as a leader when looking at our appointed lesson here. In this reading, Peter comes to Jesus seeking clarity on what seems a simple question. "How many times should a person forgive a brother or sister?"
By this time in Matthew's telling of the story, Peter seems to be catching on a bit about Jesus. He knows that Jesus is a Messiah and doesn't think or act like other people, so he shows the master that he is learning. If he would have approached the question in the usual way, he would have said, "Shall I forgive as many as two or three times?" This would have been the rabbinic approach: to forgive, yes, but prudently. To forgive once is generous. To be let down by the same person and forgive a second time would be exemplary. To be fool enough to get hurt by the same individual a third time and to forgive even then: this is bordering on the obsessive. But, Peter knows that Jesus thinks big. He makes a bold move: "Forgive as many as seven times?" This would be absurd by rabbinic standards, but it might just be the number Jesus would like. It's a holy number, and it has the kind of exaggerated quality that Jesus likes (go the second mile, give your cloak as well, etc.). Surely, we imagine Peter thinking, Jesus will like this answer. Surely this shows strength of leadership, holy insight, generosity of spirit. Surely, this is an impressive demonstration of all that Peter has learned from the great teacher.
But, Jesus disappoints him. "Not seven times, but, I tell you, seventy-seven times." Nice try, Peter. You are moving in the right direction. You just have a long way to go, yet. Four hundred ninety. This is the number, if you have to have a number, but it is an impossible number. Completely unthinkable. Jesus uses an absurd exaggeration. What it means is this: forgive your brothers and sisters beyond your ability to keep track. If you are keeping track, it is not really forgiveness at all. You may seem to be kind, but keeping track simply means that you are waiting for your neighbor to cross some line--generously drawn, perhaps--but a line nonetheless. Beyond the line you are no longer willing to forgive. Jesus calls into question the entire game. If you keep count, it is not called forgiveness.
Then, to reinforce the lesson, Jesus tells this parable. Note again the amazing exaggeration. The servant owes ten thousand talents to the king. Calculating this number (always tempting to do in a sermon on this parable) is nearly impossible. One quickly finds that Jesus is using a number that has no realistic present day equivalency. Further, no king would loan an incalculably large amount to a servant. But, that is what this king is willing to loan and to forgive. When the servant then fails to forgive another for a remarkably small debt, his forgiveness is withdrawn and the king has him thrown into jail. Again, exaggeration seems to be the way Jesus makes his point here. He is using hyperbolic language to teach Peter and to teach us about the true nature of forgiveness.
But, there is a great irony in this. Peter heard hyperbole in Jesus' answers, but, he could not know that ultimately, this was the size of debt Jesus would soon forgive. All the sins of all believers in all the world through all of time. An unbelievably large debt. This is the way to conceive of the debt of ten thousand talents owed the king. There is no way that Peter, the one who tried to stop Jesus from facing toward sure crucifixion in Jerusalem ("God forbid it Lord," 16:22), could foresee that this was the debt Jesus was to pay on the cross. The King of creation would pay with his life, so that the incalculable debt would be erased. Peter ultimately came to see this and preached this message to the church founded upon his leadership.
The lesson for readers today is still the same. Forgive without calculation or reservation. Of course, this is not easy to do. Each failure to do so is another talent piled onto the sin-bearing cross. The life of discipleship is a balance: we do not aggrieve the king to pile up our debt simply because we know it will be forgiven; we love and offer forgiveness even when we feel unable because the forgiving king loves through us. Failing to do so is to challenge the king to reverse his grace and substitute the judgment we deserve.