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
script.
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.