The grain that is the primary ingredient in the Indian flatbread roti is wheat.
Traditional roti is made from atta, a finely milled whole-wheat flour. The flour is mixed with water to form a soft dough, which is divided into balls, rolled into thin rounds, and cooked on a flat griddle called a tava. Roti is normally unleavened, so it rises little or not at all compared with breads such as naan.
Across the Indian subcontinent, roti is a staple accompaniment to vegetables, lentils, and curries. It is often used to scoop up food or carry it to the mouth. The word “roti” can also refer broadly to numerous regional flatbreads, so recipes vary considerably.
Some regional varieties use other grains, including millet, sorghum, maize, or rice. Bajra roti is made with pearl millet, jowar roti with sorghum, and makki di roti with maize. These are genuine roti varieties, but wheat remains the standard ingredient in the plain roti or chapati described by the question. Naan is another common mix-up: it is typically leavened and often baked in a tandoor.