Tax sale certificate
The lien a buyer gets at a tax sale for paying a property's overdue taxes. The owner can clear it by redeeming, which means paying the amount plus interest and a 2%, 4% or 6% penalty. If it isn't redeemed, the holder can seek foreclosure.
New Jersey example
Say an investor buys a certificate on your home at 9% interest. Taxes keep coming due, and if the holder pays them, those amounts earn interest too. To redeem, you pay the certificate amount, the interest and a 2%, 4% or 6% penalty based on the certificate's size. After two years, the holder can start a foreclosure case in Superior Court.
Official source
Elements of Tax Sales in New Jersey, NJ Division of Local Government Services
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.