This is the 342nd most frequent Japanese word.
なかなか
"なかなか" means "quite," "considerably," or "not easily" (often context-dependent).
Here, 'なかなか' is used to mean 'rather' or 'quite' to indicate that the level of difficulty is notable.
この宿題はなかなか難しいですね。
This homework is rather difficult.
In this context, 'なかなか' is used with a negative verb to mean 'seldom' or 'not easily'.
彼はなかなか学校に来ない。
He seldom comes to school.
Here, 'なかなか' makes an affirmative statement to express that something is considerably or surprisingly good.