Taking a Fourier Transform of an image simply converts the image into a new representation. This representation is useful because it can be reversed (by an inverse FFT), and because it breaks the image down by spatial frequency. The Fast in FFT refers to a special algorithm which calculates the Fourier Transform very efficiently.
FFTs can be used to very efficiently implement a spatial filter. They can also be used to analyze the content of images. MaxIm DL includes tools for both types of uses.