A day on which terrorists (supposedly from the Al-Quaeda network but what do we know, it's our media after all) hijacked 4 planes. 2 hit the World Trade center Towers, toppling them. Another one hit the Pentagon. The fourth was surmisedly aimed toward the Pentagon as well, but the passengers re-hijacked and crashed it. Overall, 3000 people died. Some people say it was a conspiracy. Some believe the government's story. Some tell people to shut the hell up and stop grieving because they say they had their chance to grieve, while in reality they just didn't get affected directly (how do they know the one talking about September 11 wasn't?). Yet more make a point of the fact that the U.S. does far worse things. My opinions don't matter. What does matter is the repercussions and how our government handled it, shouldering off all the aid attempts from the UN after asking with open arms for assistance, as well as to start a couple oil wars. But I digress.
In summary, 9/11 stands for september 11th, 2001. That was the date of the terrorist attack. It symbolises different things to a lot of people. My only oninion I'll stick in is that people should keep their views to themselves. You could really offend someone talking about that, and possibly get your ass kicked to Nebraska.