To put it as easily as possible, frequency separation is a Photoshop technique which separates the texture of an image from its shape creating two layers, one storing color and luminosity information and the other one, storing the detail. Activating a curve adjustment layer, restricted to operate on the high frequency layer, you can change the detail level of the image by steepening or flattening the curve. You will also be able to easily clone over defective parts with no visible side-effects.