1what — the basic interrogative pronoun, read なに, used to ask about a thing, event, or content.
何を勉強していますか。
What are you studying?
今晩は何を食べたいですか。
What would you like to eat tonight?
何か飲み物を買ってきます。
I'll go buy something to drink.
箱の中に何が入っていますか。
What's in the box?
2you-know-what; that thing — a casual vague reference to something both speakers understand without naming it.
この前話していた、何、あの件はどうなった？
What happened with that thing we talked about before?
冷蔵庫に、何、昨日買ったケーキがあるよ。
There's that cake I bought yesterday in the fridge, you know.
3whatsit; what's-his-name — a casual filler used when a person's or thing's name will not come to mind.
あの人、何、田中さんの友達だよ。
That person — what's-his-name — is Tanaka's friend.
何、あの映画の題名を思い出せない。
What's-it-called — I can't remember the title of that movie.
4A crude colloquial euphemism for the penis, usually written ナニ.
下品な冗談で「ナニ」と言い換えていた。
In a crude joke, he used ナニ as a euphemism.
5With a negative, it means not at all or cannot really say, chiefly in 何とも〜ない. In modern Japanese, this expression is normally read なんとも.
6what?; huh? — an interjection of surprise or a request to repeat something.
「明日、試験があるよ。」「何？聞いてないよ。」
「There's a test tomorrow.」「What? I hadn't heard that.」
何？もう一度言って。
Huh? Say that again.
7A sharp what? or what's that? expressing irritation or anger.
何、文句あるの？
What, do you have a problem with that?
何、急に大きな声を出して。
What? Why are you suddenly raising your voice?
8oh, it's fine — an interjection that lightly dismisses an apology or concern and reassures the other person.
「遅れてごめん。」「何、気にしないで。」
「Sorry I'm late.」「Oh, don't worry about it.」
何、こんな傷ならすぐ直るよ。
Oh, a scratch like this will be fixed in no time.