A colloquial exclamation expressing surprise, confusion, or displeasure—like “what's up with…?” or “what the hell?”
何なの、その態度は？
What's with that attitude?
今の音、何なの？
What was that noise just now?
彼、急に泣き出した。何なのよ。
He suddenly burst into tears. What the heck is going on?
そんなに怒ることないよ。何なの？
There's no need to be so angry. What's up?