This is a big deal because Greece's economy was in bad shape for a while, and they had to borrow a lot of money. By paying some of it back early, they're showing they're getting better off financially. It should also make it easier for them to borrow money in the future.
This comes after Greece already paid back some loans early to the International Monetary Fund (IMF) and other European countries. They're hoping this will help them get back to a normal financial situation.