From "A Reference Grammar of Modern Italian" (Maiden and Robusttelli, 2000, p. 143) "che" and "cosa" and "che cosa" all three mean "what" in English. You see "cosa" more in the north and "che" more in the south, but there's little difference between them.
So you can think of "che cosa" as a single word that means "what" and think of "che" or "cosa" as abbreviations of that. (Che actually has a few restrictions when used alone, but let's not go there.)
According to google translate (not the best source to use) "cosa" as a noun means "thing". However, I think "what thing" is apart of "che cosa" and "che cos'è". Thing is omitted because it's already assumed that you're asking about a something. So in essence "che cos'è populare" should really be "what (known or unknown 'thing') is popular"
Of course I could be wrong because my command of the Italian language is between novice and intermediate at best, yet It does make sense though.
Most of the discussions here are about the same "cosa" haha! So how would someone ask, "what thing is popular?" --> "Che cosa cos'e populare?"NOPE! In spanish "Que cosa es popular?" would literally translate to "what thing is popular?" and if you wanted to say "what is popular you would ask, "Que es popular o quizas qual es mas popular de todo, todo depende en que lo que quieres comunicar." Literal translation versus intended translation, I don't think it's wrong to translate things literally. Cause the goal here is not to learn english, but to learn italian, and it helps to learn it literally with the knowledge that you wouldn't use it necessarily in that form in english.
"Popolare" is one of those adjectives which are the same for masculine and feminine. The plural is always "popolari":
- il popolare
- la popolare
- i popolari
- le popolari
If you didn't know that, there's another hint in this sentence: "è" is singular, so no doubts about that. The sentence is actually asking "What does the word popolare mean?", so it has no semantic gender either. It's not about a specific thing which could be described with a word having its gender.
That would be "Che cosa vuol dire populare?" There's an old expression "to mean to say" that simply turned into "mean" in English ("What does antidisestablishmentarianism mean to say?" --> "What does antidisestablishmentarianism mean?") but in many other languages (Italian among them), they still use the full "to mean to say" (volere dire).
Another way of saying it "What does popular mean?" would be "Cosa significa populare?" or "Cosa populare significa?" which is related to the English word "signify" and may be more commonly used for inferences from known facts than for meanings of words, though I couldn't say for sure. An example might be: "Mario non vuole giocare con te." "Significa non gli piaccio?" (Translation: "Mario doesn't want to play with you." "Does that mean he doesn't like me?")