It is definitely slang. ButBut I believe it is more synonymous with Seriousserious, or Seriouslyseriously... "Is he for real?" or Person A: "I just jumped over that garbage can" Person B:"For real?" That is the best explanation I have for you. But it
"Is he for real?"
or
Person A: "I just jumped over that garbage can."
Person B: "For real?"
It is simple, if you are using this specific slang you are probably in a very informal setting.