A more thorough explanation:

A Qualified Domestic Trust (QDOT) is a type of trust that is created to allow a surviving spouse who is not a U.S. citizen to qualify for the marital deduction. This deduction helps to keep the decedent's estate from being taxed federally at the time of their death.

For example, if a U.S. citizen is married to a non-U.S. citizen and they pass away, the non-citizen spouse would not be eligible for the unlimited marital deduction that a U.S. citizen spouse would receive. However, if a QDOT is created, the non-citizen spouse can still receive the marital deduction for assets within the trust.

The QDOT is governed by the Internal Revenue Code, which states that a surviving spouse who is a U.S. citizen is not required to pay tax on the estate of the first deceased spouse. However, a non-citizen surviving spouse does not qualify for this exemption and must instead use a QDOT to retain the marital deduction.