What does the parable of the unmerciful servant mean?

In Matthew 18 Christ says the kingdom of heaven is likened to a certain king who would take account of his servants, and the king is Christ. One servant owed him ten thousand talents, the debt of all the sins of his life. When he fell down and asked for patience, his lord was moved with compassion and forgave him the debt. It is all wiped out, and that part everybody understands.

But that same servant went out, found a fellow servant who owed him about a thousandth of what he had owed, took him by the throat, and cast him into prison. When his lord heard it, he said, O thou wicked servant. It is the servant who has been forgiven who holds a grudge, and the lord delivers him to the tormentors. This is hell, not purgatory. And Christ ends: so likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.

It is what he also teaches in the Lord's Prayer, forgive us our debts as we forgive our debtors. In the same way that we forgive other people, we are asking the Father to forgive us.