Minor note. If the original data is a time-signal like in electrical engineering (amplitude vs. time function), then the "frequency domain pixels" (its transform) are different frequencies (points in frequency domain: how many repetitions in a second, etc.) and the time-signal's transform function becomes an amplitude vs. frequency graph.

But if the original data is an image (matrix or grid of pixels in space), then the "frequency domain pixels" are different wave-numbers (aka spatial frequencies: how many repetitions in a meter, etc.) and the Fourier transform (of the pixel grid) is a amplitude vs. wave-number function.