FFT is an acronym for Fast Fourier Transform. Use this tag for FFT-related questions. Try to also select a specific chip or software tag when possible.
Some articles about the fft for learning more:
http://en.wikipedia.org/wiki/Fft
http://en.wikipedia.org/wiki/Discrete_Fourier_transform
http://www.dspguide.com/ch12.htm
http://www.complextoreal.com/chapters/fft1.pdf