Because it's used in some greetings ... I bet you know that now though. Habari za kazi?
I thought the same, but asking how someone's job is, is a very polite sentence to say. :)
'Kazi nzuri' is a response to greeting I recall... So I believe this is leading to that.
This is the first time I've seen this word and I couldn't press on it to see the meaning.
When you see something like that later, report that a hint is missing. Comments here usually don't help in making corrections to the course.