The what and why of Passover kashrut is pretty clear: the five grains known as chametz (wheat, oats, rye, barley, and spelt) are banned, as they could become naturally leavened when combined with water; and leavening is forbidden in commemoration of the Exodus, as the Jews ate unleavened bread as they fled Egypt. But many Jews abstain from an entirely different class of food, though the reasons are less clear.
Kitniyot are the foods that are not chametz, but are traditionally prohibited during Pesach. These include rice, corn, millet, some seeds, and legumes, including beans, lentils, soy, and peanuts.
According to the Orthodox Union, which supervises the world’s largest kosher organization, kitniyot are forbidden because they could seem similar to chametz, especially when ground, creating confusion:
“The Vilna Gaon (1720-1790) and Peri Chadash (1586-1667) found a basis for not eating kitniyot in the Talmud (Pesachim 40b). The Gemara relates that Rava did not allow the use of lentil flour on Pesach in a Jewishly unlearned community, as he feared it would lead to confusion and cause one to mistakenly eat chametz on Pesach.”
For example, wheat flour (chametz) could be confused for rice flour or cornmeal (kitniyot). Additionally, the “unlearned community” argument suggests that allowing something like cornbread while banning other bread would be “confusing.”
Aish, a Jewish outreach organization, explains further that rabbinical writings have expressed concern that chametz could easily be mixed in with kitniyot.
Observant Ashkenazi Jews almost universally abstain from kitniyot; but observant Sephardi Jews generally eat kitniyot throughout Pesach.
However, kitniyot are not treated entirely like chametz. Chametz traditionally should be sold and completely removed from the house, but kitniyot need not be. So you can keep that jar of peanut butter in your pantry, unless you won’t be able to resist the temptation to nosh.