2 Nephi 18

Isaiah taught -

"Bind up the testimony, seal the law among my disciples."  (Book of Mormon | 2 Nephi 18:16)

Interesting enough, modern revelations states

"...bind up the law and seal up the testimony..."  (Doctrine and Covenants | Section 88:84)

Isaiah added -

"...should not a people seek unto their God for the living to hear from the dead?
20 To the law and to the testimony; and if they speak not according to this word, it is because there is no light in them."  (Book of Mormon | 2 Nephi 18:19 - 20)

In my mind, prophets that wrote scripture are they that received the law and taught it.  And then, having obeyed the law, bore testimony of its truth.  This is the same for all men -

"If any man will do his will, he shall know of the doctrine, whether it be of God, or whether I speak of myself."  (New Testament | John 7:17)

In my mind, this is what bind up the law and seal up the testimony or vice versa means.