GST Tax is "Generation Skipping Transfer" tax. It is another federal-level tax. It is another transfer tax imposed on direct gifts and transfers in trusts to or for the benefit of "skip" persons. While this can be a very complicated area of taxation, in general, you can think of this as gifts to your grandchildren's generation or farther apart in age. The government wants a chance to charge the estate tax at each generation, so if you "skip" one of those generations, this tax steps in to let them impose an estate-tax substitute and capture that lost revenue.
There are many pitfalls and traps when trying to leave gifts to grandchildren or more remote recipients if your estate exceeds the GST tax exemption.
The GST-tax exemption amount currently mirrors the federal estate tax exemption as of the time of writing; however, this is not always the case. Before making any gifts to grandchildren (or equivalent younger generations) in trust of any amount, or any gifts outright to grandchildren (or equivalent younger generations) over the annual gift tax exclusion ($17,000 in 2023), you should consult with this office or another estate-planning lawyer before making any such gifts.