本日のランチの内容はなんですか。

today

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

lunch; launch; ranch

subject; contents; matter; substance; detail; import

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

what; how many; you-know-what; that; whatsit; what's the expression; what do you call them; what?; hey!

be; is

indicates a question (sometimes rhetorical); or; whether or not; some- (e.g. something, someone); indicates doubt, uncertainty, etc.; emphatic prefix; suffix forming adjectives or adverbs; in that way