(idiomatic) An informal greeting, not requiring a literal response. In form a question, and thus followed by a question mark. Typical responses include:

Short for (may) God bless you: said as a short prayer for the recipient. / (idiomatic) Said to someone who has just sneezed, as a polite remark.

(colloquial, chiefly Canada, US, Australia, New Zealand, South Africa, Britain) You (plural).

(idiomatic, Britain, colloquial) An intensified version of what or huh.

(US, Midwestern US and Appalachia) You (plural, as subject or object).

Used other than figuratively or idiomatically: see fuck, you. / (idiomatic, offensive, markedly vulgar) Expression to show discontent with the other party, or to show contempt.

(vulgar) An insulting and humiliating action against somebody.

(informal, idiomatic) Used, often after a short list, to show that further examples are unnecessary; all kinds of things.

(Wales) see here

(idiomatic, colloquial) Used to express the speaker's skepticism about a preceding statement concerning some desired or assumed state of affairs.