Ha, I know someone would ask! Closing is a feature that I turned off because it doesn't work quite how I'd like. I just turned it on for a second to close that question, for the hell of it.
When it's turned on there is just a "close" button next to "hide" that essentially locks the question from new posts. The idea is so that new users can see what they did wrong instead of their question just disappearing.
I haven't got around to changing it to how I want it to work. It will probably work like this: you click "flag" and it gives you some reasons to flag the question (duplicate, spam, rule-breaking), then when enough flags have been given, it closes the question. Then some time later the question will be hidden.