The lime of nixtamal
Cal is calcium hydroxide, a fine white powder from Mexico, 4 oz to the bag. It is the lime of nixtamal: dried corn cooks and soaks in water with cal, and only then is it ground into masa for tortillas and tamales.
Cal is what gives a tortilla from your own nixtamal its soft, pliable bend, the one that lets you roll and fold it without a crack. In the US you also see the same compound sold as pickling lime or slaked lime. Four ounces is a home kitchen bag, enough to try nixtamal at home without buying a sack. Read the handling note on this page before you open it.
In the YERBERO catalog, Cal para nixtamal is filed under C in the herbolario A–Z, next to Flores de caléndula.