This is an adverb that people sometimes use instead of "very". It's not exactly slang, because people of all ages use it. But it is used when you want to sound a little bit "different". It has a more energetic and excited feeling than "very". Common words that people use after "super" include:
He's super rich.
Everyone there was super cool.
It's super easy to make. (Often used when on cooking TV shows and in recipes.)
He's not only strong, but super fast as well.