A debtor is commonly known as a company, entity or individual that owes you money.
There can be many types of debtors that you may have within your business. Debtors can include your customers, refunds of taxes, vat etc, prepayments of amounts e.g. rent or any loans you have given to other parties.
A debtor is the opposite to a creditor. If you are a debtor the other side of the transaction is the creditor.