In terms of images.
Fourier transform converts spatial to frequency (Fourier) domain. DC value = average of sinusoids (sine waves), F(0,0) and average brightness/graylevel of image. Fourier has a complex number output....you can get the [magnitude and phase] or [real and imaginary] outputs.
What I don't get is what the complex number output actually represents when you do FFT? I know an image in the fourier domain is the sum of weighted sine waves but what does the output actually mean.
What is spatial frequency? When I look at definitions it states it as the rate of change of pixel values., what does this mean?
So you can visualise it better if you parameterise the complex number information to a magnitude and phase value, it respresents the magnitude of the frequency component and phase gives you the position of the component in the image.
Spatial frequency typically refers to change in intensity value when you traverse across pixels. So an edge in your image would have a high spatial frequency due to the sharp change in pixel values