Matzo (also Matzoh, Matza, Hebrew: מצה), an unleavened bread, is the "official" food of Passover. When the Jews were leaving Egypt, there was no time for the bread to rise, and the resulting food was matzoh. For Passover, the ingredients for matzoh are flour and water.
Five grains are forbidden for use during Passover in any form but matzoh: wheat, barley, spelt, rye, and oats. (Wheat and spelt are both in the genus Triticum and anything else in the genus is likewise forbidden.) Millet and teff are borderline; it takes a few days for them to rise. Dough made from the five grains is considered to start rising 18 minutes from the time it gets wet; if longer elapses before it is put in the oven, it is no longer matzoh. Shmura ("watched") matzoh is made from grain that has been under special supervision from the time it was harvested to ensure that there was no additional moisture.