According to ABC News, pink slime or BLBT (Boneless Lean Beef Trimmings) or finely textured lean beef, “is more like gelatin than meat” and something that was sold “only to dog food or cooking oil suppliers” until they “found a way to use it by disinfecting the trimmings with ammonia”.
ABC News also says that
The low-grade trimmings come from the parts of the cow most susceptible to contaminaton, often close to the hide, which is highly exposed to fecal matter. But because of BPI’s treatment of the trimmings — simmering them in low heat, separating fat and tissue using a centrifuge and spraying them with ammonia gas to kill germs — the United States Department of Agriculture says it’s safe to eat.
It’s worthwhile reading the article, especially if you want to avoid purchasing such a product, since it is not easy to recognize it. If you want to be safe, then “If your meat is stamped USDA Organic, it’s pure meat with no filler”.