Total Tax from last year's return = would be your prior year tax liability. The IRS safe harbor rule (110% of prior years Total tax liability) is there to allow for an individual taxpayer that has no idea what their total tax liability will be until the end of the tax year to just pay estimated taxes in the current year to equal 110% of the prior year total tax liability. It is a kind of loop hole to avoid the interest penalty, but the taxpayer still owes the balance of tax liability of the current year above the estimated taxes paid.
