The Credit Shelter Gift is designed to shelter an amount equal to the available estate tax exemption equivalent so that no federal estate tax is ever paid. The idea is that an amount equal to the exemption equivalent is either left to your descendants (outright or in trust), or, it can be placed in trust for the life of your spouse and then transferred to your descendants (either outright or in further trust), where, in either case, it will escape taxation in the estate of the surviving spouse.
This technique can be utilized during life or at death. Of course, if utilized during lifetime, the tax leverage can be many times greater, but there is a price: you have to be in a position to afford to part with the money, since you lose the right to use it after the gift is made. (If your spouse is a beneficiary, however, you may obtain some indirect benefits from a lifetime gift.)