In Mormonism, the sealing power is the means whereby all "covenants, contracts, bonds, obligations, oaths, vows, performances, connections, associations, or expectations" attain "efficacy, virtue, or force in and after the resurrection from the dead." (D&C 132:7.) All things that are not sealed by this power have an end when men are dead.
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Restoration
This power is believed to have been held in Old Testament times by the prophet Elijah, and that he restored this power to Joseph Smith in the Kirtland Temple of the Church of Jesus Christ of Latter Day Saints on April 3, 1836 in fulfillment of an Old Testament prophecy in Malachi 4:5-6:
Marriage
An ordinance where the sealing power is clearly displayed is in marriage. A typical marriage ceremony includes the caveat "until death do you part", whereas marriages performed with the sealing power in a temple, can exist beyond death and into heaven. These marriages are referred to as celestial marriages, and are for eternity, not just until death. In this instance, husbands and wives are referred to as being sealed to one another, and the children are sealed to their parents, making an "eternal family."
Detailed explanation
The sealing power is explained in greater detail in D&C 132:7-19:
Boyd K. Packer has also elaborated on the meaning of the term "seal" in conjunction with the phrase "sealing power":