Masa harina is a very finely ground corn flour made from corn that's dried, cooked in water with slaked lime (which gives it distinctive flavor), ground, and dried again. Mixed with water (or sometimes oil), it forms the dough called "masa" that is used to make corn tortillas.
To get that distinctive flavor, try grinding stale or dry corn tortillas in a food processor until you get a fine powder. If the masa harina is used as a thickener (such as in chili), regular flour or cornmeal will give you the right texture, though the flavor won't be quite the same.
For more information on ingredient substitutions, see our Ingredient Sustitutions guide.