Ketzah is the Hebrew name for black cumin, which was thought to have medicinal properties to detoxify the body of various ailments, as well as cosmetic properties to smooth and beautify the skin and complexion of women.  In addition, as the novel indicates, the seeds of the ketzah (cumin seeds) are “hot to the tongue.” These details, as well as the fact that it grows in a fertile valley, carry thematic significance. Ketzah as a place is fertile while the caves and rocks of the mountains, where Daniel lives with Rosh, are not. It is in this site of fertility that Daniel matures by working and caring for his sister, acts of nurturance, which correspond to fertility.  He, like the plant, has medicinal properties in that through love he helps to cure Leah of her “demons.”  That “ketzah” as a plant produces a spicy seed speaks to Daniel’s character, for his tongue is as hot as the seed. He often speaks and acts out of anger—he has a fiery temper that he must learn to subdue over the course of the novel.

You can find the answer in chapter 3 of the book. For more details, see the enotes discussion of the setting of the book, which states that "the village takes its name from the ketzah plant, whose blue flowers yield seeds that are sold as food seasoning. This village is the home of the novel's main character, Daniel bar Jamin." Follow the link below.