"What is important?"
Translation:Was ist wichtig?
I'm a native German and I've only ever heard it used when modifying another adjective (ie, wesentlich groesser, wesentlich wichtiger) similar to how you would use the English word "significantly". Having it as an adjective by itself sounds very strange to me. "Was ist wichtig?" is a much better translation.
maybe it's used as adjective in philosophy. "wesentlich" means essential, and "essential" is what makes something to be what it is. At least, Heidegger uses it in this way:
"Nähe und Anwesenheit, nicht die Grösse des Abstandes, ist wesentlich"
Indeed I don't know whether common folks in Germany talk like that :)