Credit default swap
When people found out that the Greek government owed more money than everybody thought it did, people who didn't own any Greek government bonds started buying credit default swaps on Greek bonds. They did that because they thought Greece would give up on trying to pay, so the bonds would become worthless and the people who sold the credit default swaps would pay them. Unfortunately, this makes people who have Greek bonds nervous, so they want to sell them and not buy any more. That makes it hard for Greece to borrow money to fix its money problems.