What does portability of the estate tax exemption mean?

Portability lets a surviving spouse use whatever portion of the deceased spouse's estate tax exemption went unused. With the exemption at 15 million dollars per person, a couple can effectively shelter up to 30 million dollars combined. To claim it, the executor must file an estate tax return after the first spouse dies, even if no tax is owed. It's a valuable option for wealthier couples, but most families never need it.