What is a “superfood”?

I browsed a healthfood store last week and one of the things on the shelf was a "superfood". I didn’t ask a worker (they were pretty packed and I had no intention of buying–was just browsing). Does anyone know what this is and what its benefits are? (Hearing superfood sort of equates that with some anti-kryptonite ingredient in my mind, so a little reality injected here would be greatly appreciated.)

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