Fundamental if you ever finished a CS degree. Shamefully, the Wikipedia articles (German and English) aren't really good, and I dropped out of university before FFT was a topic.
Lots of HN readers are self-thaught hackers or come from non-theoretic backgrounds... so they likely won't have had a good explanation of it. (Speaking of which, the circle analogy is truly awesome - but how does it work in code?!)
I'll just mention http://www.dspguide.com/ as a really fantastic resource - a free book aimed at scientists and engineers but so well written as to be easily accessible to the interested lay person.
A lot of the math-related articles on Wikipedia are terrible. When you look at the page for the Math project the FAQ basically says 'it's an encyclopaedia, not a textbook, go write on the talk page of individual articles.' The project managers seem hyper-defensive about it, in constrast to the comprehensive and inclusive approach of many other fields. The Wikibooks project is not much better (so far).
I find the Wolfram Mathematica pages more accessible for dealing with new concepts.
Lots of HN readers are self-thaught hackers or come from non-theoretic backgrounds... so they likely won't have had a good explanation of it. (Speaking of which, the circle analogy is truly awesome - but how does it work in code?!)