This is a rarely used tactic, but it is used when the IRS has exhausted every other option at their disposal and you have continued to ignore them. They will do a public records search for all assets you own including real estate, cars, boats, motorcycles, jewelry, etc.
Then, once they locate these assets, they will take possession of them and sell them at auction typically at half of their value. The amount they collect is used to pay their costs incurred to seize and auction the assets and the remaining balance is applied to your tax debt.
If you have received a seizure notice, do not wait to find professional help! You are in immediate need of professional representation.