This is not strictly a Textual Criticism issue – but it’s a related topic and it is linked to Leviticus, where I’m doing a bit of work at the moment.
If you have ever read much about Hebrew OT, or read technical OT commentaries, or tried to decipher a page of BHS, then you have probably come accorss the terms qere and kethiv (or kethib). This is what they are:
BHS is a copy of the Leningrad Codex (a complete Hebrew Bible dated to 1008/9AD. It is the oldest complete copy of the OT that we have. This codex was written by a group of Jewish scribes who saw it as their role to preserve the Hebrew OT. So these guys worked like horses to do everything in their power to ensure that the text of the OT was never lost. To do this, not only did they memorise and write out the text, but they also developed ways to write down the vowels and accents for the text. (They developed ingenious ways of doing this so that they didn’t have to alter the consonantal text, which was sacred to them. That’s why in BHS and other Hebrew bibles, the vowels and accents are written above and below the consonants, but rarely ever between them.)
Because the Masoretes held the consonantal text as sacred, they never changed it – but they did spot some mistakes, or varient readings. And so they developed another ingenious way of showing this without having to change the letters: they made a distinction between qere (“what is said”) and kethiv (“what is written”).
An example to explain can be found in Leviticus 9:22.
What is written (kethiv) in the text is ידו (yod-dalet-waw) but what is read (qere) is ידיו (yod-dalet-yod-waw). If you look carefully, you will see a small circle over the word in the main text which is the Masoretes way of referring you to the outer margin where you will find a ק (the symbol for qere) with some other consonants written over it.
What the Masoretes did was to maintain the original consonants (ידו) but to write the correct vowels underneath those consonants, while keepiing the ‘correct’ consonants in the margin. So what is written is ידו but what is read is יָדָיו. The kethiv is the singular noun ‘hand’ with a 3ms suffix (“his hand”) whereas the qere is the plural noun + 3ms suffix (“his hands”).
The Wikipedia article has some good info on different types of qere-kethiv.