When you're in debt, and you go further in debt For example, say you got a new charger the other day, and your parents bough it but you need to pay them back, so they owe you $15. Then, your headphones break and they buy you a new pair for $30. You're already $15 in debt from them, and you owe them $30 more, so -15 - 30 = -45.
