Each state is different. In principle you are paying taxes for a property that is in default. So the state can still get its money to pay for its services. In exchange, you have a lien on the property that bears interest. When the owner eventually pays the taxes you earn the interest. If the owner does not pay the taxes within a certain time, you can start a foreclosure and in some state you will own the property, in other it will go to auction and the winner will pay your taxes back with interest.
As a whole tax lien are very secure investments (the only risk is if the property is destroyed and the lien value become higher than the vacant land. So make sure that you don't overpay for your lien). Some states are more secure than others.
They need a lot of good research work before buying but can get some good returns. The most usual reason to lose money in tax liens is failing to find other municipal liens, or not underwriting the property. Many people bought strips of useless land 1ft wide between highway, or wetlands... It is also a very competitive market and it may be tough to find good ones.