The Tax Relief Unemployment Insurance Reauthorization and Job Creation Act was signed into law in December 2010. This made significant changes to federal estate taxes, generation skipping transfer taxes, and gift taxes. 
This was the first time that the potability of an estate tax exemption was introduced. Portability is a concept that is only available to married couples. The amount of an estate tax exemption that is not used for a deceased spouse’s estate can be transferred to the surviving spouse in the event that the first spouse passes away and the value of their estate does not use up all of the exemption.
This then enables the surviving spouse to use the deceased spouse’s unused tax exemption in addition to their own exemption when the surviving spouse passes away. This is a federal level exemption and state level exemptions are only available in two locations, Hawaii and Maryland as of 2020.
The federal estate tax exemption is $11.58 million for those deaths that occurred in 2020. Exemptions are amounts that are subtracted from the total value of an estate and only the balance that remains after this is applied is subject to the estate tax.
Because of the substantial size of this in the United States, it is very rare for many people to trigger the estate tax amount. For more information about this might apply to you and various estate planning strategies you can use, set up a time to meet with an estate planning attorney.