Please Read p.1-25 of the PDF below on FFT basics.

"Now that we can view the image frequency data, how do we interpret it? Each pixel in the spectrum represents a change in the spatial frequency of one cycle per image width. The origin (at the center of the ordered image) is the constant term, sometimes referred to as the DC term (from electrical engineering's direct current). If every pixel in the image were gray, there would only be one value in the frequency spectrum. It would be at the origin. The next pixel to the right of the origin represents 1 cycle per image width. The next pixel to the right represents 2 cycles per image width and so forth. The further from the origin a pixel value is, the higher the spatial frequency it represents. You will notice that typically the higher values cluster around the origin. The high values that are not clustered about the origin are usually close to the u or v axis."