A dished face is a concave profile of the horse's head, sometimes with a bulged forehead. Extreme dishing is most often seen in Arabians, but many pony breeds also have slightly dished faces. In Arabians, the dish is desirable and associated with stamina - it's assumed that it has something to do with having the right airway for endurance work in warmer climates. (Pony dishes are much less extreme).
This Arabian only has a slight dish, but the forehead bulge is visible. Image source Ealdgyth via Wikimedia commons.