The human body requires 40 nutrients, and some of them have to be in balanced amounts relative to the others. For example, sodium makes cells absorb water whereas potassium makes them dump water. So the exact amount of either is not critical as long as the body gets approximately equal amounts of both. Some number of those nutrients are proteins and the body can make a few but there are eight proteins that you have to eat. The correctly balanced combination of those eight is called complete protein. For example, beans and corn combine to make complete protein. There are other such combinations, but for convenience we only count proteins from meat, milk and eggs because those are sources of complete proteins each by itself. If you get an incorrect combination the liver has to break down the excess and it gets used as ordinary but expensive calories.
So which of those forty nutrients do you think makes a "superfood"?
Bottom line: "Superfood" is a marketing slogan.