Here is a site which explains it: http://www.familiesforever.com/poligamy_mormonism.html A marriage ceremony in a Mormon temple, called a sealing because it is performed with the sealing power that binds in heaven what is bound on earth, joins a couple in marriage for time and all eternity.

The practice, as we understand it today, was revealed. This revelation can be sound in the Doctrine and Covenants. The term "seal" I think is most properly explained in D&C 132:7 "And verily I say unto you, that the conditions of this law are these: All covenants, contracts, bonds, obligations, oaths, vows, performances, connections, associations, or expectations, that are not made and entered into and sealed by the Holy Spirit of promise, of him who is anointed, both as well for time and for all eternity, and that too most holy, by revelation and commandment through the medium of mine anointed, whom I have appointed on the earth to hold this power (and I have appointed unto my servant Joseph to hold this power in the last days, and there is never but one on the earth at a time on whom this power and the keys of this priesthood are conferred), are of no efficacy, virtue, or force in and after the resurrection from the dead; for all contracts that are not made unto this end have an end when men are dead." Basically this means that anything done on earth by earthly authority is only valid on earth. (Earthly authority doesn't have heaven in its jurisdiction.) If we want to make a covenant that is valid in heaven, it must be done by God's authority. Verses 15 and 16 read: 15.) Therefore, if a man marry him a wife in the world, and he marry her not by me nor by my word, and he covenant with her so long as he is in the world and she with him, their covenant and marriage are not of force when they are dead, and when they are out of the world; therefore, they are not bound by any law when they are out of the world. 16.) Therefore, when they are out of the world they neither marry nor are given in marriage; but are appointed angels in heaven, which angels are ministering servants, to minister for those who are worthy of a far more, and an exceeding, and an eternal weight of glory. The marriage covenant similar to baptism (but not exactly the same) is essential for exaltation. (D&C 131 is also instructive on this point.) Like baptism for the dead, sealings are also done, here on earth, on behalf of the dead. Sealings to spouse are done in temples.