The "jot" and "tittle" refer to the smallest letter and a mark which distinguished two letters in the Hebrew words of the Old Testament. Not only the words, but even the letters of scripture are divinely inspired.

Source:

KJV Defenders Study Bible, by Dr. Henry Morris, Ph.D.
Publisher: Thomas Nelson
Language: English
DEF 10 ISBN 0-529-10444-x
DEF 10-1 ISBN 0-529-10445-8