(idiomatic) Precisely what the situation really is or what the facts truly are; the true state of things.

What is. / What has. / What does.

(colloquial, sometimes rhetorical question) What is wrong with. / (colloquial, sometimes rhetorical question) What is the reason for.

(colloquial) What is that? / (colloquial) What was that?

Alternative form of know what is what

Alternative form of what's what

(UK, dated) Used to add emphasis to, and to seek agreement with, a previous statement.

(euphemistic, African-American Vernacular) What the hell.

(interrogative) Which, especially which of an open-ended set of possibilities. / (relative) Which; the ... that. / (relative) Any ... that; all ... that; whatever.

(interrogative) Which thing, event, circumstance, etc.: used in asking for the specification of an identity, quantity, quality, etc. / (fused relative) That which; those that; the thing(s) that.