Animal by-products include organs (liver, heart, kidney), bone, blood, and other parts excluded from human food. They're not necessarily low-quality nutritionally — organ meats are nutrient-dense and would be highly valued in a wild diet. The concern is transparency: "chicken by-product meal" tells you the source species; "animal by-product" or "meat by-product" doesn't, which is what makes the latter worse from an ingredient-quality standpoint.