When someone in Maryland cannot, or refuses, to pay their property tax, the government imposes an obligation on them: pay up or face foreclosure and repossession. That is what the Tax Lien represents, that threat of foreclosure and repossession. (Lien essentially just means debt; you can translate Tax Lien as a Tax Debt)
However, to get the property tax income they need and not deal with the hassle of chasing after reluctant taxpayers, each Maryland county sells these Liens to private investors. This allows the government to collect the money quickly and the private investors to make significant returns (or even obtain the property).
Once the Tax Lien is established by the government, they offer it up for purchase in a Tax Sale auction as a Tax Lien Certificate. This is what you can purchase, and it gives you the right to reclaim the property after a specific amount of time has passed if the owner does not pay both the original tax amount plus additional fees.
