The document discusses the Fast Fourier Transform (FFT) algorithm. It explains that the FFT decomposes an N-point discrete Fourier transform (DFT) into smaller DFTs of size N/2, taking advantage of the periodicity and symmetry of complex numbers. For N that is a power of 2, it separates the input sequence into even and odd indexed parts, then recursively applies this decomposition until 2-point DFTs are reached. This decimation-in-time approach reduces the computational complexity from O(N^2) for the direct DFT to O(NlogN) for the FFT.