First of all, for what reptile is this regarding?
Well, sources of protein for herps are usually, but not limited, animal matter; in other words, meat. This also includes insects, and certain vegetables, if I'm not mistaken. And some of the above have higher protein-content than others. For instance, superworms have higher-protein than mealworms, which are higher in fat and chiten. Also, rodents have a little higher fat content than, lets say, canned monitor/tegu food.
A savannah monitor fed nothing but mice and eggs would be said to have a high-protein diet. It's like us living on steaks and hamburgers; it keeps us going, but that high-carbs and cholesterol will catch up to us. Savs primarily dine on invertebrate prey in the wild, and rarely come across the treat of having a rodent for lunch.
So likewise, I suppose, some herps would have a low-protein diet. Green iguanas would such an animal, as well as any other herbivores. Definitely research your animals and their natural diet so you can better simulate it in captivity. I hope that helps a little.
