What does Matthew 18:24 mean?

Jesus is telling a parable to illustrate why His followers should plan to forgive each other, always (Matthew 18:21–22). He has begun by saying the kingdom of heaven can be compared to the king of a large nation. That king was ready to settle accounts with his servants or slaves. This mean that his slaves owed the king money.

One of the servants who was brought before the king owed him the astronomical figure of 10,000 talents. Depending on how one calculates things like inflation, gold prices, and so forth, the modern value of this figure can change quite a bit. Generally speaking, a talent was about as much money as a low-level laborer could make in twenty years.

The sum Jesus mentions here is so large that it becomes meaningless to calculate an exact number—this is literally many thousands of lifetimes worth of debt. What's owed is exponentially beyond what the debtor can ever hope to pay. Likewise, how any servant could ever come to owe his king this much money is impossible to imagine. That, also, is not the point of the story. However, it also factors into the king's merciful nature that he allowed the servant to go so far into debt in the first place.

Jesus might even have been using a deliberately exaggerated figure simply to make His teaching clear (Matthew 18:25–27).