Ancho chiles (or anchos) are whole poblano peppers which have been dried. The term translates from Spanish to English as "wide". The ancho is part of the "holy trinity" of chiles used in mole sauces. In California ancho chiles are frequently referred to as pasilla chiles.
The ancho does not pack a lot of heat, in fact on a relative scale from one to 10 the ancho is only about a 3-5. See the scale here. This makes this chile the perfect addition to soups and stews or any dish where a little heat and flavor is required.
This is one of the more common chiles and can typically be found in the Hispanic food section of well-stocked grocery stores, or any Hispanic grocer. You can also purchase them here, online at GourmetSleuth.com. See the buy link this page.
If you don't have ancho chiles you can substitute equal amounts of: