Ancho powder is ground dried poblano pepper (ancho chile), used as a spice in Mexican and Latin-influenced cooking. Ripe poblanos are dried - traditionally in the sun - until wrinkled and reddish-brown, then ground fine, concentrating their natural sweetness and earthiness into a mild, complex, faintly spicy powder.
It's a key ingredient in mole, adobo, and enchilada sauce, and is also used more broadly to season roasted or grilled meats and vegetables, and in spice rubs and dry brines.
Find it in the spice aisle or Latin markets, usually alongside chipotle, guajillo, and pasilla powders. Look for a deep reddish-brown color and a fragrant, earthy aroma; store in an airtight container away from light and use within a few weeks for best flavor.