Pronounced roughly "MAHN-isht-ahn-ah", this Hebrew saying originated in a religious context: it's the beginning of first of the four questions asked during Passover, which asks "How is this night different from all other nights?" Colloquially, it is used to express utter lack of surprise at a supposed piece of news. It's a way of saying "Tell me something I don't know" or "What else is new?" with a snarky urban Jewish twist.
Late with taxes? Ma nishtana! Girlfriend dumped you because you played video games too much? Ma nishtana! Your junkie friend is in rehab again? Ma nishtana! etc.