I will give an example of a sentence which I could'nt understand at all: "Upstream dams and levees built to tame Mississippi River floods and ease shipping have starved the delta downstream of sediments and nutrients, causing wetlands that once buffered the city against storm-driven seas to sink beneath the waves". This sentence has a lot of words I did'nt kwow, and, even when I looked for it in a dictionary, I did'nt get it's complete sense.

In fact, this whole paragraph was difficult for me to understand, because of my absence of some prior knowledge. For example, I did'nt know that New Orleans is part of Louisiana, near to Mississipi River, and that one of it's nicknames is The Big Easy (!). So, the informations on the third paragraph seemed confused. When I looked out for New Orleans at Wikipedia, I could understand the connections between the informations.

To do the article's summary, I had to understand the text - that is, to imagine what is the article's major goal and the smaller goals in its parts. And I believe that we only can imagine the writer's intention when we have sufficient world knowledge and when we can undestand most of the words of the text. That's why I considered this task so difficult.