To make valid in heaven the ordinances performed by priesthood authority on earth. Ordinances are sealed when they receive the approval of the Holy Spirit of Promise, which is the Holy Ghost.

Whatsoever thou shalt bind on earth shall be bound in heaven, Matt. 16: 19 (Matt. 18: 18; D&C 124: 93; 132: 46). Ye were sealed with that Holy Spirit of Promise, Eph. 1: 13. I give unto you power, that whatsoever ye shall seal on earth shall be sealed in heaven, Hel. 10: 7. To them is power given to seal both on earth and in heaven, D&C 1: 8. Those in the celestial glory are sealed by the Holy Spirit of Promise, D&C 76: 50-70. Elijah commits the keys of the sealing power into Joseph Smith's hands, D&C 110: 13-16. This is the sealing and binding power, D&C 128: 14. The more sure word of prophecy means knowing that one is sealed up unto eternal life, D&C 131: 5. All covenants not sealed by the Holy Spirit of Promise end when men are dead, D&C 132: 7. The great work to be done in the temples includes the sealing of children to their parents, D&C 138: 47-48.