I'm not sure what them mean by ash on the label, either.
On the one hand, ash is high in minerals - calcium and phospherous mostly, and I've seen claims that adding up to 1% wood ash into your poultry feed can be of benefit. There's a popular belief that ash increases protein digestibility. Most of what I've read suggests that not the case, and in the case of meat and bone meal, higher ash% correlates with a reduction in some of the amino acids (like lysine) chickens most need.
More likely, however, its talking about the gross mineral content if you cook the feed at very high temp, such that all the moisture, fats, carbs, and proteins cook away, leaving only things like calcium, phosperus, iron, etc...