Masa harina is corn flour made from sun-dried field corn (maize) that has been prepared with slaked lime or wood-ash lye water, which softens it and enables it to form a dough. After it's cooked, the posole is left overnight to soak in the lime water, and then it's ground into masa and dried.

Masa harina is not the same as cornmeal. It's softer, absorbs more fat, and holds together better than cornmeal.