Question: What is the meaning of “the nothingness of the children of men; yea, even they are less than the dust of the earth?”
Answer: In essence, men are less obedient than the dust of the earth. Elder Jeffrey R. Holland said, “Hills, mountains, seas, earth---all obey His voice. But man does not obey His voice; in willfulness and pride he tramples under foot the words of the Holy One.”
“O how great is the nothingness of the children of men; yea, even they are less than the dust of the earth. For behold, the dust of the earth moveth hither and thither, to the dividing asunder, at the command of our great and everlasting God” (Helaman 12:7-8)
But it should be remembered that men have agency---and as a result, they have potential that mountains and dust never will.
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 171.