Debtor's Prison was where a man was sent when he could not pay his bills. Not a member of the peerage, mind you. They could not be forced to honor their debts if they were of a mind not to. Although the law did not exactly exempt a peer from prosecution for nonpayment of a debt, it was unlikely that a creditor could actually prevail. As a result, few titled gentleman actually feared incarceration. For the common gentleman, however, debtor's prison was a very real threat.
Common prisons for debtors of the time were the Fleet (left) and the Marshalsea (shown below).