The estate tax marital deduction is used to reduce the gross taxable estate of a decedent for purposes of calculating estate tax. It generally is the value of property that passes from a deceased spouse to a surviving spouse (this includes the deceased spouse’s one-half share in community property in community property states).
Be aware, however, the marital deduction is not available for property passing to a non-US citizen spouse unless the property is held by a special trust, referred to as a “QDOT” (Qualified Domestic Trust).
The theory behind allowing the marital deduction property to pass estate tax free to the surviving spouse is straightforward… the government will get its tax when the surviving spouse dies, so it does not need to tax the property on the death of the deceased (i.e. first to die) spouse.
The above is a vast over-simplification of a complicated topic, but it gives you an idea about the marital deduction for estate tax purposes.