What does it mean when your writing topic is too narrow?

In order to understand this you have to understand what a writing topic is and how it can be broad or narrow. The subject of your piece is its topic; your topic is what you are writing about. You might start with "Stuff people do" which is impossibly broad. You could narrow that to "Rituals people do", or further to "Marriage rituals", or further to "Marriage rituals in Europe" or again to "Marriage rituals in France", or "Marriage rituals in France in the 15th century", but when you have narrowed it to "Marriage rituals in 1474 in the LaForge family in Besancon, France" you might only be talking about one event or even none, and you will have nothing to say. So, while a very broad topic like "Stuff people do" leaves you with too much to say, an overly narrow topic leaves you with not enough to say.