Monetization of debt means that the bonds representing the debt are converted into cash. Typically this term is used for government debt, but business debt can also be monetized.
Corporations sell bonds. Doing this gets people to risk the money they paid for the bond while they earn current interest. Hopefully, the buyer will receive back their purchase funds.
This means that the government prints money to pay debt rather than relying on taxes or other revenue sources. It causes inflation, which is a tax on money.