understanding, comprehension / appreciation

essence (inherent nature of a thing or idea)

understand, comprehend / appreciate

important, significant

importance, consequence, significance

question, problem on homework etc. / a question; a matter (of time etc.) / a problem (something that needs to be dealt with); a trouble / an issue; a matter

importance

impossible to understand, utterly confused

(informal) no problem

unthinkable, out of the question