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1.
A term used to give an indirect importance to a noun. The word "it" obviously refers to what the person is referring to in general context as in:

"The Club? Down two streets, and take a left. On Sherbrooke street is where it's at."

But, it can be used when "it" hasn't been specified and the user merely intends to give it prime importance, such as:
"Simple Plan? No way, they suck big cock. The Rasmus is where it's at now."
by Comrade Karl July 14, 2006