Practically, this could take the form of central banks buying government debt or keeping interest rates low for a longer period of time than it would be necessary to ensure price stability. Then, traditional roles are reversed: monetary policy stabilises real government debt while inflation is determined by the needs of fiscal policy.
For monetary policy to deliver price stability, it is ultimately dependent on sustainable fiscal policy. This is why acting as a lender of last resort for governments can prove a slippery slope. If governments can expect to be bailed out by central banks, chances are that they will adjust their behaviour accordingly.