Total debt is the sum of everything a company has borrowed and pays interest on: loans, bonds, leases, and revolving credit. It does not include everyday operating bills like accounts payable, since those do not charge interest.
Set against equity, it shows how much the company leans on borrowing (the debt-to-equity ratio). Set against EBITDA, it shows how many years of operating earnings it would take to pay the debt off (debt divided by EBITDA, used in credit analysis, where below 3 times is generally considered safe).