Tax lien is a certificate of lien on an unpaid tax property put by Internal Revenue Service (IRS). When a property owner fails or refuses to pay their taxes, they becomes a delinquent taxpayer and a governing authority is in charge of collecting the taxes. However, if the authority is not able to collect the required property taxes, they will opt to take the final step to collect the taxes. The public auction is the final step that will take the delinquent properties to be sold.
Source: tax lien investing