This is the amount of the loan on the date you last signed a mortgage or other security instrument (deed of trust, etc.) for the property. That may be the amount and date of the purchase money mortgage, or it might be the amount and date of a refinancing. Whatever was the most recent event with respect to that loan will provide the details to be filled in here.
Because the Tax Cuts and Jobs Act of 2017 made changes to the way deductible mortgage interest is determined, these additional questions have been included to allow TurboTax to help you get the correct deduction.