Debt on date last secured refers to the loan amount on the date that you most recently signed the papers for the loan (mortgage or deed of trust, promissory note). This may be the date on which you actually purchased your home, or a later date if you have refinanced your mortgage loan.
Fair Market Value on date debt was last secured by home would be the property market value as of the date of the refinance. This can be determined through methods like an appraisal, comparative market analysis, online valuation tools, or tax assessments.