It's a person with a higher concentration of tastebuds per square inch than average. There's a wikipedia page that's incredibly sparse. As far as I can tell, science hasn't really focused on it enough for there to be anything thorough on it.
It's as real as that genetic quirk that can make cilantro taste like soap.
There are little tests they sell online but I think you can do it without it. You use a little bit of blue food dye on your tongue to make your taste buds easier to see. Then you punch a hole in a piece of paper (I forget what size, 1/4 hole punch maybe?) and if you have more than a certain amount of tastebuds in that hole you're a super taster. You're gonna have to google it to get the specific numbers. I'm on mobile so it's not worth for me to look and then come back and edit this.