Ancho pepper flakes are simply dried poblano peppers ground. The poblano pepper is native to the state of Puebla, Mexico and they are a type of mild chili pepper. Though green ancho chiles are mild and robust in flavor, ripe red chiles can be incredibly hot and flavorful.
The poblano is a mild chili pepper called ancho or chile ancho, from the Mexican Spanish name ancho or chile ancho. Stuffed fresh and roasted it is popular in many Mexican recipes.