I found this sentence 'occupy Wall Street protests'on the web forum and I don't exacty know what it imply - especially the meaning of 'occupy' in this sentence.
Please help me!
The actual protest is officially called "Occupy Wall Street". Look here.
The sentence you saw should have had a capital "O" on Occupy and refers to the protests that are taking place as part of the whole thing. "Occupy" means to enter/take possession of a building without permission as a protest. When a group of people goes into a building and refuses to leave because they are trying to make a political (or other) point, they are said to be "occupying" the building.
In this case they are "occupying" a small park in New York City, for no apparent reason other than dissatisfaction with their economic status.