From Bible History Online :
The "Jot" is the Hebrew word "Yodh"
which is the 10th letter of the Hebrew alphabet. It is also the smallest letter.
It's European or English equivalent is the letter "Y" as in the English term
Yahweh or in Hebrew YHVH since there were no vowel's used in the ancient
The word "jot" itself is an English transliteration of "iota" which is the 9th letter of the Greek alphabet. "Iota," in turn, is the nearest Greek equivalent for the Hebrew yodh.
The "tittle" is the small decorative spur or point on the upper edge of the yodh. If you can imagine a tiny letter with a slightly visible decorative mark.
Tittle is used by Greek grammarians of the accents and diacritical points. It means the little lines or projections by which the Hebrew letters differ from each other. One example would be the difference between the letter L and I. The difference is only one small mark. We use phrases like "the dotting of the i, and the crossing of the t," and "every iota."
It is interesting that the Jewish scribes who copied the MT (Massoretic Text) of the Hebrew Bible scrolls paid the greatest attention to the minutiae of detail and such marks attached to each consonant throughout the entire text. They even numbered every letter, word, sentence, paragraph, chapter, section, and scroll to insure that the total equalled that of the text being copied before allowing it to enter the holy synagogue.
The meaning of the passage is very clear. Not even the smallest letter or even its decorative spur will ever disappear from the "God Breathed" Word until all is fulfilled. In fact when heaven and earth are replaced by a new heaven and earth, the Word of the Lord will have accomplished its purpose and will be fulfilled in every detail even to the very letter.