This is the 10248th most frequent Japanese word.
難し
"難し" (むずかし) means "difficult" or "hard."
Here '難しい' is used as an adjective to describe the challenging nature of the problem.
この問題は難しいです。
This problem is difficult.
In this sentence, '難しい' is as an attributive adjective to describe the complexity of the question.
彼は難しい質問をしました。
He asked a difficult question.
This instance uses '難しい' to express subjective difficulty in understanding something.