A high-maintenance person is basically a person who needs a lot of attention and/or material things to be happy. It's kind of hard for me to explain.

I think we talked about it in class before.

If you take a look at the original meaning of "high maintenance" you will notice that if something needs high maintenance, you have to pay your more attention and maintain it all the time. If your car is high-maintenance, that means you have to repair or maintain it constantly. So it also means the same thing when it refers to people. If you still didn't get it clearly, I would have to say, "high maintenance people" are “难伺候的人” in Chinese.