An unsecured creditor is a creditor that lends you money simply based on your promise to repay rather than requiring you to pledge collateral as security in case you default on the debt.
For example, a credit card is typically an unsecured debt. The creditor only has their ability to use the law to enforce repayment of the debt. A debt to a friend is an unsecured debt. The friend only has your word that you will repay.
On the other hand a home mortgage is a secured debt since the home is pledged as collateral for the repayment of the debt. If the payments are not made then the creditor has the authority to repossess the home to recover the debt owed.