Question: What is the meaning of the jot and tittle metaphor?
Answer: “For verily I say unto you, one jot nor one tittle hath not passed away from the law, but in me it hath all been fulfilled” (3 Nephi 12:18).
A jot, or yod, is the smallest letter in the Hebrew alphabet. A tittle is a tiny literary mark or stroke that distinguishes similar letters from each other.
The metaphor assures the chosen Twelve that every word spoken by the ancient prophets will be fulfilled–-even the smallest portion of the law, for God is the same yesterday, today, and forever, and will not vary a jot or a tittle. (See Matthew 5:18)
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 182