I believe he's referring to the DCT transform, in which the image is interpreted as a mixture of sinusoidal waves. Low frequencies then correspond to difference

I believe he's referring to the DCT transform, in which the image is interpreted as a mixture of sinusoidal waves. Low frequencies then correspond to differences between separate areas of the picture, while high frequencies correspond to texture. I'm not a big fan of the periodic transforms, but they do have that nice perceptual interpretation.