Definition:
Secured debt refers to debt which is protected or "secured" by collateral. This means that if the debtor defaults on the debt, or in other words, fails to pay back the debt, the lender can look to the collateral for repayment.
When filing for bankruptcy, you must list all of your creditors holding secured claims on Schedule D.
Examples:
If you take out a loan to purchase a car or a house, the debt that you owe the lender is secured. It is secured by the collateral, specifically, the car or the house.