Tax sale
An auction a town holds at least once a year when property taxes or town charges go unpaid. The buyer pays what is owed and gets a tax sale certificate, which is a lien. The property itself is not sold.
New Jersey example
Say a home has $4,000 in unpaid taxes and interest. At the tax sale, bidders compete by offering to accept a lower interest rate, and a certificate can earn no more than 18%. The winner pays the town the $4,000 and gets the certificate. If the rate is bid all the way down, bidders offer a cash premium instead. The owner still owns the home but must pay off the certificate to clear the lien.
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.