When learning to read Torah, all you need to do is memorize the notes and the vowels, right? Well, not always. Sometimes you have to change the letters themselves!

There are words in the Torah that we don’t pronounce as they are written. In these places there is a קְרִי (keri, spoken form) and a כְּתִיב (ketiv, written form).
One of these is in our parashah. The Torah talks about how if a person sells a house in a walled city, they have the right to purchase it back from the buyer for one full year. But if that year ends, and they didn’t buy it back, then it belongs to the buyer forever, for as long as they want to keep it. Here’s what you might find if you look closely at a printing of the Torah reading that talks about this kind of house:
הַבַּ֨יִת אֲשֶׁר־בָּעִ֜יר אֲשֶׁר־[ל֣וֹ] (לא) חֹמָ֗ה
the house in the walled city
Something funny is going on here. The word לא (lo) means “no.” That word appears without vowels on it, showing that it isn’t meant to be read; it is the ketiv. But the next word, לוֹ (lo), which here means “has,” does have vowels, showing that it is the keri.

What’s the difference?

The ketiv seems to mean “a house in a city that doesn’t have a wall”; the keri means “a house in a city that has a wall”—the exact opposite! What’s cool is that these two very different words aren’t even pronounced differently. It’s all about what you are thinking as you are reading.
Why write it one way and say it another? Rabbi Elazar son of Rabbi Yose (in Sifra Behar, section 4) says it is meant to teach that even if the city no longer has a wall, but it did when the house was sold, the one year rule still applies. So you could have a house in a city that sort of has and sort of doesn’t have a wall!
If all of this sounds strange, you should remember that the most common keri/ketiv split in the Torah is God’s name itself. We can’t even write it out here fully because it is so holy, but when the four-letter name of God appears in the Torah with the letters yod-heh-vav-heh, we don’t pronounce those letters, but rather say “Adonai” if we are reading it in shul, reading a whole pasuk, or saying a prayer (otherwise, we don’t even say that, and we say “Hashem” or something else instead). So there are a huge number of cases of keri and ketiv all over the Torah!