fft is a simple 2d FFT on images. In forward transform, it takes single images in input and produces a cube containing two images in output: either (real,imaginary) or (amplitude,phase). The reverse transform works exactly the other way round.
For the moment, fft only supports images which size is a power of 2. If anyone has a request for other image sizes in astronomy, drop a mail to the authors. There are FFT libraries covering readily any kind of signal size, but no one has ever requested such a functionality. Furthermore, there is an FFT command in virtually any data analysis environment so this command is really there for completeness.