"Current Outstanding Debt" refers to debts that are owed by the tenant (the tenant hasn't paid) and these debts were there before the system was adopted by your company.
On this page you can enter the Current Outstanding Debt also called the "Old Debt" by registering that old amount in the field here called "Current Outstanding Debt"

These debts can be paid after the system has been adopted and they can be Cleared when you go to the page "Enter Rent Payment" and click "Clear Old Debt"