The expression used here has reference to the day of Christ’s mortal ministry, usually designated as the meridian of time. Using the same expression as Nephi, Paul wrote,
“When the fulness of the time was come, God sent forth his Son, made of woman, made under the law, to redeem them that were under the law, that we might receive the adoption of sons.”
(Galatians 4:4-5).
Paul also spoke of “the dispensation of the fulness of times” as the day in which we live- the day in which all things are to be restored
(see Ephesians 1:10).
Those living before Christ’s earthly ministry would properly see his coming as a time of fulness or a time of completion not only of the law of Moses but also of thousands of messianic prophecies.
In the revelations of the Restoration the phrase is used to identify our dispensation as the fulness of all past dispensations (see D&C 27:13; D&C 121:31; D&C 124:41; D&C 128:18, 20). The priesthood, keys, powers, and authorities of all past dispensations can be likened to rivers emptying into the ocean of fulness in which we live.