By-products will normally consist of lower quality ingredients- necks, feet, bone, blood, stomach and other tasty cast-offs. Not a quality source of protein.
By-products can be a great source of beneficial nutrition, if they consist of specific internal organs like liver or kidney from organically fed animals. This is rarely the case.
If you come across a food which lists organ meats, you’re looking at by-products in your pet food. Nothing to be concerned about, but it is an example of how this ingredient can be embraced in one formula while demonized (rightly so) across most of the industry.