Basically, it is a tax paid on the assets you transfer at the time of your death, with the "estate" primarily responsible for the tax.
From the IRS's website:
The Estate Tax is a tax on your right to transfer property at your death. It consists of an accounting of everything you own or have certain interests in at the date of death. The fair market value of these items is used, not necessarily what you paid for them or what their values were when you acquired them... The total of all of these items is your "Gross Estate." The includible property may consist of cash and securities, real estate, insurance, trusts, annuities, business interests and other assets.
Once you have accounted for the Gross Estate, certain deductions (and in special circumstances, reductions to value) are allowed in arriving at your "Taxable Estate." These deductions may include mortgages and other debts, estate administration expenses, property that passes to surviving spouses and qualified charities. The value of some operating business interests or farms may be reduced for estates that qualify.
After the net amount is computed, the value of lifetime taxable gifts (beginning with gifts made in 1977) is added to this number and the tax is computed. The tax is then reduced by the available unified credit.
Most relatively simple estates (cash, publicly traded securities, small amounts of other easily valued assets, and no special deductions or elections, or jointly held property) do not require the filing of an estate tax return. A filing is required if the gross estate of the decedent, increased by the decedent’s adjusted taxable gifts and specific gift tax exemption, is valued at more than the filing threshold for the year of the decedent’s death, as shown in the table below.
And the table they are referring to is this:
|Year of Death||If Amount Described Above Exceeds:|
On January 1, 2026, unless the United States Congress passes a new tax law, the amount in the table is set to change to a $5,000,000 figure that will be adjusted for inflation. This amount will certainly be lower than the amounts for years 2018 through 2025.
Important note: this a Federal tax, not a state tax. States can have their own estate taxes as well, but fortunately neither Texas nor Michigan at the time of writing have a state-level estate tax.