A debtor is someone who owes money. If you borrow from a bank to buy a car, you are a debtor.
Most of us are debtors at some point in our lives. We borrow money to buy houses or cars, to attend college, or to tide us over when we're between jobs. Businesses and large institutions can also be debtors, and even countries are often debtors. If a developing country borrows money from a wealthier one, the borrower is a debtor. The opposite of a debtor is a creditor.