A tikkun is a book used by Jews to prepare for reading or writing a Torah scroll. There are two types of tikkun, a tikkun kor'im and a tikkun soferim.
A tikkun kor'im (readers' tikkun) is a study guide used when preparing to chant the Torah reading from the Torah in a Jewish synagogue. Each tikkun contains two renditions of the masoretic text in Hebrew. The right side of each page is written with the cantillation marks and vowel points, while the left is written in unpointed Hebrew, as it is written in the actual scroll. People who chant from the Torah must learn the tune and the pronunciation of the words beforehand, as the scroll itself has neither points nor cantilation marks, and since there are places where the word to be read (the Qere) differs from that written (the Kethib) in the scroll.
A tikkun soferim (scribes' tikkun) is similar, but is designed as a guide for scribes writing a copy of the Torah by hand. It contains additional information of use to scribes, such as directions concerning writing particular words, traditions of calligraphic ornamentation, and information about spacing and justification. For instance, it helps the scribe to know how many letters there are per line, so a tikkun soferim gives the size of each line, measured in yud-widths (because yud is the smallest Hebrew letter).