What is High Maintenance, really?

One of my favorite movies is "When Harry Met Sally". I believe that there are so many quotes that I can relate to, esp. during the last 10 years of my life. So, anyway, Billy Crystal points out that there are 3 types of women: high, low, and then the worst. . . high that thinks that they are low maintenance. I am a high maintenance woman. Yet, what does that REALLY mean?