In many places in the United States of America, a homeowner owes the government a yearly tax on their property. If the homeowner does not pay this in the specified deadline, the Internal Revenue Service or IRS has the authority to issue a tax lien on that property.
If the homeowner fails to pay the required taxes after the redemption period, the government would seize the property and sell it to the public. The prices of the property included in the tax lien sale would almost be dirt cheap in these circumstances. This allows a wise investor to come in and buy low, then sell high later on.