What (it) is; asking what something means or what something is about (used to inquire or express puzzlement/curiosity).
これは一体何なのか、誰か説明してほしい。
What on earth is this? I want someone to explain it.
彼の言っていることが何なのかよく分からない。
I don't really understand what he means.
問題の原因が何なのか、調べてください。
Please investigate what the cause of the problem is.