FFT stands for fast Fourier transform.
It’s widely used in movies (pretty much every movie where there is a CG ) and in AAA titles such as assassin’s creed and GTA5.
wiki quote:
“The FFT algorithm computes the discrete Fourier transform (DFT) of a sequence, or its inverse. Fourier analysis converts a signal from its original domain (often time or space) to a representation in the frequency domain and vice versa.”
The inverse FFT transforms a spectrum from the frequency domain which would look something like :
…to a heightfield in the spatial (image) domain which looks something like :
In more simple terms it basically lets you compute the sum of several hundred thousands of waves 