Okay, so imagine you have a piggy bank where you save all your pocket money. Now imagine you lend some of that money to your friend who promises to pay you back later. But then, your friend spends all the money and can't pay you back. Now you don't have enough money to buy your favorite toy or candy.
That's kind of what happened in some countries in Europe. They lent a lot of money to their friends (other countries) who promised to pay them back later. But some of these countries spent all the money on things they couldn't really afford, like fancy cars or big houses.
When it was time to pay the money back, they didn't have enough. This made it hard for the countries that lent the money to get their money back, and everyone got worried that something bad might happen. This caused a lot of problems and is known as the European debt crisis.